# ytplayer — project facts for agents

## What this is
Ad-free YouTube player. Three shells share `frontend/`: **web PWA** (Bun + Hono in
`server/`, the production deployment), Tauri (Windows), zero-native (Linux/macOS).
The PWA is what runs in production; `legacy/` holds the old native-only docs.

## Repo map

| Path | What lives there |
|------|------------------|
| `frontend/app.js` | The entire UI (~3.6k lines, no framework): player, playlists, search, settings, sync |
| `frontend/sw.js` + `frontend/sw-update.js` | Service worker + update flow (FRAGILE — see below) |
| `frontend/opfs.js` / `opfs-worker.js` | OPFS offline audio cache |
| `frontend/fingerprint.js` | Browser fingerprint used as the sync key |
| `frontend/async-guard.js` | Stale-async-response guard (unit-tested) |
| `server/server.js` | Bun + Hono backend — endpoint list is in its header comment |
| `server/db.js` | libsql schema/queries (`users`, `profiles`, `video_history`, `media_cache`…) |
| `server/media-cache.js` | Server-side video cache: background fetch jobs, validation gate, x264 lane, LRU |
| `bin/yt-dlp` | Downloaded by `npm run setup`, gitignored |
| `scripts/` | icon generation, yt-dlp setup, push helper |
| `tests/` | Playwright e2e specs · unit tests live next to sources in `frontend/*.test.js` |
| `legacy/` | Old native-shell docs — do not treat as current |

API endpoints (notes/admin routes are listed in the header of `server/notes.js`): `GET /api/search|channel|streams|download/:id|version|user/data|profile/load`,
`GET /api/media/:id|media/:id/status|media/stats`, `POST /api/media/:id/redownload`,
`POST /api/user/sync|profile/create|profile/save`, `GET /sw.js` (BUILD_TAG-injected), `GET /*` static.
JSON shapes mirror the Tauri Rust bridge exactly — don't change one side alone
(`/api/streams`' `data.serverCached` is additive and web-only).

## Server media cache (`server/media-cache.js`)
- Every played (`/api/streams`, LOW priority, ≤ `MEDIA_AUTO_MAX_SECONDS`) or saved
  (`/api/download`, HIGH, ≤ 3 h) video gets ONE copy: `$MEDIA_DIR/<id>.<gen>.mp4`
  (≤720p H.264 8-bit + AAC, faststart) + `<id>.<gen>.m4a` audio sidecar for audio-only
  mode. `MEDIA_DIR` defaults to `./data/media` (the `ytplayer-data` volume in prod).
- Jobs are server-owned — **never pass a request AbortSignal into them**; a client
  closing its tab must not kill a fetch. Downloads still run inside `withSaveSlot`.
- **Validation gate** (`validateMedia`): h264 yuv420p + AAC, container/stream durations
  cover the source, a full `-c copy` demux pass with zero stderr, head + tail decode.
  The demux pass is what catches a faststart file whose index is intact but whose
  data is cut. Nothing reaches the cache dir without passing it; failures back off
  (15 min · 2^n, max 24 h) and saves fall back to the legacy per-request path.
- `gen` is in the filename and URL (`/api/media/<id>?g=<gen>`): a URL never serves
  different bytes. A replaced gen is kept 30 min so in-flight Range playback survives.
- **Compression lane** (rev `OPT_REV` = 2): niced x265 8-bit `-preset medium -crf 28`,
  `hvc1` tag, anime-style tuning (`X265_TUNING`), `-fpsmax 30`, audio copied — kept only
  if ≥15% smaller, and the copy's `vcodec` becomes `hevc`. **Sample-first**: videos ≥60 s
  get two 10-s windows encoded first and compared with the source's own packet bytes
  over the same timestamps; the full encode only runs if the prediction clears the 15%
  bar (a skip costs ~20 s of video; a full encode ~2× realtime on the homelab).
  **Why:** re-encoding YouTube's already-compressed H.264 usually needs MORE bits for
  LOWER quality (generation loss) — on prod 0 of 5 videos shrank at CRF 28, one grew 25%
  (VMAF 93.6 at 126% of source; capping to 60–70% drops VMAF to ~84–87). Only videos
  where YouTube over-spent bits win (a live worship video: −34%). AnimeOut-style sizes
  come from pristine Blu-ray sources, which YouTube never gives us. Bumping `OPT_REV`
  re-queues every cached copy once at boot.
- **HEVC gating**: an HEVC copy is served only to clients that send `?hevc=1` on
  `/api/streams` and `/api/download` (app.js `hevcCapable()` = canPlayType hvc1, minus a
  per-device `ytpNoHevc` flag set when an HEVC copy fails to play). Everyone else gets
  the pre-cache behaviour (YouTube proxy / legacy H.264 save). Verified: iPhone XR, Galaxy
  S10, Windows Chrome play HEVC; AV1 fails on the XR (no decoder), so it isn't used.
  The validation gate requires `hvc1` (Safari refuses `hev1`) and 8-bit 4:2:0.
- **Budget**: `MEDIA_CACHE_MAX_BYTES` (10 GiB) LRU by `last_access` (anything played in
  the last 10 min is protected) + `MEDIA_MIN_FREE_BYTES` (5 GiB) disk guard → skip, stream.
- **Broken**: `POST /api/media/:id/redownload` (now-playing ⚠ Broken button). When a
  client falls back from a cached copy (`/api/streams?nocache=1`) the server also
  re-validates its copy and refetches it if it fails — never trusts one device's error.
- Ops: `curl https://worship.hesed.sbs/api/media/stats`; files under
  `/app/data/media` in the container.

## Lyrics · chapters · bookmarks (`server/notes.js`, `frontend/lyrics-core.js`)
- **Shared per video** (every user sees the same copy): `video_notes` holds the live
  lyrics / chapters doc per `(video_id, kind)`; **every save** also lands in
  `video_note_revs` as a full snapshot (history + undo). A daily JSON dump of all
  live notes goes to `<DB dir>/backups/notes-YYYY-MM-DD.json` (30 kept).
- **Who writes**: a user linked to an online profile (body `profile` must exist —
  same trust level as the profile API), a script with an API token
  (`Authorization: Bearer ytp_…`, only a SHA-256 is stored), or the admin
  (signed httpOnly cookie). Saves are optimistic: client sends `baseRev`, gets
  409 + `current` if someone saved in between (UI asks "Keep mine / Load theirs").
- **Personal, not shared**: bookmarks (`data.bookmarks[videoId]`) and the per-song
  sync offset (`data.lyricOffsets[videoId]`) — both ride the profile sync.
  Line shows at `t + doc.offset + personal offset` (positive = later).
- Editor text format = LRC superset: `[1:23.45] line`, `# Section`, `! band cue`,
  `@ Key G, 70 BPM` tags, multi-stamp LRC lines and `[offset:±ms]` accepted.
- Captions: `GET /api/notes/:id/captions` (preview) and token/admin-only
  `POST /api/notes/:id/lyrics/auto` pick human subs first, else the auto track in
  the spoken language (`*-orig`), never a machine translation. Auto captions of
  **music** are mostly junk ("oh", "n to") — expect to hand-fix or paste lyrics.
- **Admin**: `/admin` (frontend/admin.html, never cached by the SW) — tokens,
  edit feed with view/restore, caption injection, JSON export, API examples.
  Disabled unless `ADMIN_PASSWORD` is set (Dokploy Environment tab; compose passes
  `${ADMIN_PASSWORD:-}`). Login is rate-limited per IP.
- **Service mode → 🎵 Lyrics** (`data.settings.serviceLyrics`, `serviceLyricsAutoscroll`):
  lyrics-only view; the sung line is centred, highlighted and sized to the
  largest font that fits ONE row. Sizing is measured with a hidden real
  `.sl-line` in TWO passes — Bricolage Grotesque has an optical-size axis, so
  glyphs get relatively wider at small sizes and a single 100px measurement
  over-fits on phones. Re-fit (in place, never rebuilding rows — that would
  move the scroll position) on resize, `document.fonts` load, and +900 ms.
  Lyrics-only and the 📺 video panel are mutually exclusive.
- **Free batch transcription**: `scripts/lyrics/auto_lyrics.py` (faster-whisper
  `large-v3-turbo` int8 on CPU, no API key/credits; venv at
  `~/.local/share/lyrics-asr/.venv`). `--missing` uses `GET /api/admin/media`
  (admin or token). Measured on the 16-core laptop: ~0.3–0.65× real time;
  openai-whisper medium was ~2.5×. **`vad_filter` must stay False** — VAD calls
  sung music non-speech and returns an empty transcript. Karaoke/minus-one
  tracks have no vocals (reported "instrumental?"); their lyrics were read from
  the on-screen text (frame sampling + OCR), not audio. ElevenLabs Scribe also
  works but spends the TTS credit balance (free tier = 10k/month).
- Offline: the last-seen notes per video are cached in localStorage
  (`ytpNotesCache`, 200 videos), so saved songs keep their lyrics offline.
- **Toast cap bug (fixed with this feature):** `toast()` used
  `while (children.length > 3)` with a deferred remove — the 4th toast spun
  forever and Chrome killed the tab ("Target crashed" in Playwright, no JS
  error). Never cap with a loop whose exit depends on a deferred removal.

## Playlist transitions + waveform (`Transition` / `Wave` in app.js)
- `data.settings.transition` = `off` | `gapless` | `crossfade` (`crossfadeSec`,
  default 6); the ↦/⇥/⤨ button cycles it with a 6-s toast. Applies only to
  playlist/queue playback with a next song, not with loop-one or an A-B loop.
- A hidden **bridge `<audio>`** preloads the next song's audio ~25 s early
  (offline copy, else `/api/streams`). Crossfade starts it `crossfadeSec` early
  and ramps both volumes; gapless starts it when the song ends. Then the queue
  advances normally with `Player._handoff` set: `_startPlayback()` seeks the
  new track to the bridge's position, plays, and releases the bridge on
  `playing` (failsafe 5 s / 20 s). afterLoad skips resume/A-marker seeks
  during a handoff; `onTrackEnded` ignores `ended` while one runs.
- iOS ignores `element.volume`, so crossfade degrades to gapless there.
- **End-of-song race (fixed):** the song's last `timeupdate` arrives with the
  element already `paused` (it ended), and `ended` can beat the fade timer.
  Both used to look like "user paused" → the crossfade was cancelled and the
  next song restarted from 0. `tick()` and `onTrackEnded` (`Transition.onEnded()`)
  now finish the handoff when a fade is running at the end.
- Waveform: `GET /api/media/:id/peaks` = 400 RMS buckets (0..100, ^0.7 so
  quiet verses stay visible) from ffmpeg over the server-cached copy, memoised
  per file path (gen). Audio-only mode draws it on `#waveCanvas` BEHIND the
  range input (made transparent, still the control), + hover time tooltip and
  "⚡ Jump to the loudest part" (loudest ~5 s window). Not cached on the server
  → plain seek bar. Envelopes are kept in localStorage (`ytpPeaks`, 80 songs).
- Tested with real audio: a local server whose media_cache holds 20-s clips
  (upsertMedia rows + `<id>.1.m4a/.mp4` files) — Playwright's Chromium plays
  AAC; launch with `--autoplay-policy=no-user-gesture-required`.

## Uploads + web lyrics
- **Uploads** (`server/uploads.js`, `UPLOAD_DIR`, table `uploads`): an admin
  posts a file to `/api/admin/uploads` (multipart, ≤ 4 GB, admin cookie or API
  token). ffprobe runs ONCE and gives everything: title/artist/album tags,
  duration, whether there is a REAL video stream (a cover picture also appears
  as a video stream — check `disposition.attached_pic`), the cover itself
  (extracted to `<id>.art.jpg`; an uploaded image wins) and embedded lyrics
  tags (`LYRICS` / `lyrics-eng` / `UNSYNCEDLYRICS` / `©lyr`) which are parsed
  with `parseLrc` and saved as that song's shared lyrics (synced when in LRC
  form). Ids are `upl_<12 hex>` and are accepted anywhere a video id is:
  `/api/streams` (audio → `audioUrl` + art, video → one "Original" quality),
  `/api/download`, peaks/gif/clip (`audioSourcePath`/`videoSourcePath`), notes.
  `/api/search` puts the library first and still answers when yt-dlp fails.
  The app: 📁 Library view, `isUploadId`/`isMediaId` helpers, audio uploads
  force audio mode so the cover art shows where the video would be.
- **Web lyrics**: `POST /api/notes/:id/lyrics/web` asks **LRCLIB** (free,
  key-less, often SYNCED) using a cleaned title/artist + duration
  (`cleanTitle`/`cleanArtist`/`pickLrclib`, ± 6 s). Profile users, tokens and
  admins may call it; the lyrics worker tries it BEFORE transcribing.
  `scripts/lyrics/web_lyrics.py --missing --agy` adds an agy web-search
  fallback (plain text → untimed lines, to be timed with Tap-sync).
  Gotcha fixed here: a `/g` regex reused for `.test()` skips every other call
  (lastIndex) — `cleanTitle` keeps a stateless copy.

## Watch party, sharing, playback extras (all in app.js unless noted)
- **Lyrics instead of the video** (`Notes.stageLyrics`, `#stageLyrics` inside
  `.player-stage`): a per-device view for watch parties and sing-alongs —
  `data.settings.stageLyrics` / `stageLyricsAutoscroll`. It reuses the service-mode
  machinery (`fitLyricLines`, `LyricsCore.activeIndex`, the same `.sl-line`
  classes) and is driven from the normal playback tick. The `<video>` is only
  **hidden** (`visibility`), never unloaded, so playback, audio and party sync
  carry on untouched — that is why it is safe for a guest to switch views
  mid-party. Toggle from the watch-party panel or the bar inside the stage
  (hover on desktop, always visible on touch).
- **Watch party** (`server/party.js`, `Party`): `/ws/party` (Bun has ONE websocket
  handler — sockets are tagged `ws.data.hub = 'party'` and routed in server.js).
  Host drives; guests extrapolate the host's `state` from the server-stamped
  `ts` (clock offset from `hello.now`), seek when > 1.5 s off, mirror
  play/pause. Guest actions within the 1.5 s "quiet" window after a sync are
  ignored; later ones → a `cmd` (if the host allows control) or "off sync"
  (Re-sync button). `playNext/playPrev` → `Party.intercept`, and guests never
  auto-advance or crossfade. Chat is relayed (≤ 500 chars, 400 ms flood guard,
  last 60 kept). Voice = WebRTC mesh, the smaller pid offers, STUN only (no
  TURN — strict NATs won't connect). Host resumes after a reload with the
  `secret` from `hello` (party survives 5 min without its host).
- **Timestamp sharing** (`Share`): `/?v=<id>&t=<sec>` deep links (handled at
  boot via `playVideoAt`), YouTube `youtu.be/<id>?t=`, GIFs from
  `GET /api/media/:id/gif` (palettegen, ≤ 6 s, server-cached videos only) and
  soundbites from `GET /api/media/:id/clip?fmt=mp3|m4r` (m4r ≤ 40 s, written to a
  temp file so it's a normal faststart MP4 — iPhones reject fragmented ones).
- **External players** (`External`): hands out `/api/media/<id>` (the single-file
  server copy) via Android `intent:` (VLC / MX / chooser), iOS `vlc-x-callback`,
  desktop `vlc://`, or an `.m3u` (this video or the queue's cached songs).
- **Notes**: bookmarks with `kind:'note'` hold multi-line text (≤ 2000) and carry
  `vt`/`vc` (video title/channel); the 📝 Notes view lists them all, exports
  Markdown. **Transcript search** = 🔎 tab over the lyrics or the YouTube
  captions (`/api/notes/:id/captions`, cached per video).
- **PiP** mirrors the PiP window's play/pause onto the whole player (dual mode's
  sound is on `<audio>`). **EQ**: 5 BiquadFilters; media elements are wired in
  only once a non-flat curve is picked (irreversible), disabled on iOS (Web
  Audio suspends on lock → would kill background audio). **Sleep timer**: fade
  over `sleepFade` s, then pause / "close" (window.close, falls back) / black
  curtain; "end of this song" also blocks Transition. **Gestures**: double-tap
  thirds ±10 s / play, pinch 1–3× (CSS transform), vertical swipes for volume /
  brightness (CSS filter) only in fullscreen or `body.landscape-fs` (inline the
  page keeps `touch-action: pan-y` so it still scrolls).
- Local media tests: seed `media_cache` rows + `<id>.1.m4a/.mp4` files; the
  Playwright `channel: 'chromium'` build plays H.264/AAC; `--use-fake-device-for-media-stream`
  gives WebRTC a fake mic.

## Presenter view, stats, lyrics worker
- **Presenter** (`Presenter` in app.js, `/?present=<code>`): a lyrics-only
  projector/TV screen. Pairs through the phone-remote relay like a phone
  (listen-only); the host's `state` carries `v.id`, `cur`, `rate`, `paused` and
  `off` (the host's personal lyric offset), and the presenter extrapolates the
  position between the ~1/s updates. Host modal → "Open presenter window
  here" opens a popup to drag onto a second display; `/api/remote/qr/<code>?kind=present`
  is its QR. Keys: F / double-click fullscreen, B blank screen. Forced dark
  tokens so it stays readable if the device is in light theme.
- **`fitLyricLines(list)`** is the shared one-row font fitter for service-mode
  lyrics and the presenter — re-run it on font load, resize and +900 ms
  (first fit on a fresh page happens before Bricolage loads).
- **Stats** (`frontend/stats-core.js`, pure + tested; `StatsTrack` + `renderStats`):
  `data.stats.days[YYYY-MM-DD] = {s, p, songs:{id:plays}}` (local days, 400
  kept) synced with the profile. Seconds = media-time deltas while playing
  (seeks/pauses excluded, divided by playback rate); a play = 30 s heard in one
  load; commits every 60 s / on play / on page hide to avoid profile-push spam.
  Streak = consecutive days ≥ 5 min (today may still be pending).
- **lyrics-worker** (compose service, `scripts/lyrics/Dockerfile`): runs
  `auto_lyrics.py --missing --watch 300` forever — one song per cycle, nice 10,
  `cpus: 2`, `mem_limit: 3g`, audio fetched from `http://ytplayer:3000` over the
  private `lyrics` network (never the WAN). Auth = env `LYRICS_WORKER_TOKEN`
  (≥ 24 chars) accepted by notes.js as `api:lyrics-worker` without a DB row;
  unset → the worker idles. `/data/state.json` remembers instrumentals (never
  retried) and failures (backoff 15 min·2^n). Model (~1.6 GB) downloads to the
  `lyrics-models` volume on first start.

## Phone remote (`server/remote.js`, `Remote` in app.js)
- Desktop tab playing to a TV = **host**; phone = **remote**. Both are browser
  tabs of this app, so the server relays over `/ws/remote` (Bun `server.upgrade`
  in the `Bun.serve` fetch wrapper, before Hono). Mockup:
  `docs/mockups/mock-06-remote-control.html` (it assumed a LAN host + mDNS; the web
  app can't do that, so it's a server relay + a 6-digit code instead).
- Host secret lives in the desktop's localStorage (`ytpRemoteHost`); room id =
  hash(secret). Pairing (`POST /api/remote/pair`) is a one-time 6-digit code
  (10 min) → phone gets `token = HMAC(secret, remoteId)` (`ytpRemotePair`).
  **Nothing about phones is stored server-side**: tokens are re-checked against
  the connected host's secret, so server restarts / desktop reloads keep phones
  paired, and "Unpair all" (new secret) revokes every phone at once.
- The host pushes `state` (≤1/s, deduped) and `queue` (`{items, idx}` of the live
  `queue`/`queueIndex`); commands are whitelisted server-side (`REMOTE_COMMANDS`)
  and run by `runCommand()` on the host with the same functions the UI uses.
- `REMOTE_SAME_NETWORK=1` = pairing requires the phone and desktop to share a
  public IP (first `X-Forwarded-For` hop). Off by default — verify that the
  VPS→homelab Traefik chain forwards the real client IP before turning it on.
- QR: `GET /api/remote/qr/:code` (server-side SVG via `qrcode`) encodes
  `<origin>/?pair=<code>`; the app consumes and strips `?pair=` at boot.

## Local dev

```bash
npm run setup            # download bin/yt-dlp (once)
cd server && bun install
ln -s ../frontend public # once — the server serves ONLY ./public (Docker copies frontend/ there)
bun --hot server.js      # http://localhost:3000
node --test frontend/    # unit tests (run from repo root)
npx playwright test      # e2e (see Testing below)
```

Local DB file: `server/data/ytplayer.db` (gitignored). `BUILD_TAG` is computed from `./public` contents.

## Production deployment (web PWA)
- URL: **https://worship.hesed.sbs** (Traefik label in `docker-compose.yml`)
- Runs on the **homelab** Dokploy remote node; control plane is Dokploy on the VPS
  (`193.160.119.172`, API key in `~/development/.secrets/dokploy-api.env`).
- Compose ID: **`wprYCM8T51f7JtSHb983p`** (project `ytplayer`, env `production`).
- **Pushing to git does NOT deploy.** Trigger explicitly (build ≈ 5–6 min):
  ```bash
  ssh -i ~/.ssh/tmp_vps/dokploy_session root@193.160.119.172 \
    "curl -s -X POST -H 'x-api-key: $KEY' -H 'Content-Type: application/json' \
     -d '{\"composeId\":\"wprYCM8T51f7JtSHb983p\"}' http://localhost:3000/api/compose.deploy"
  # poll composeStatus via /api/compose.one?composeId=... until done|error
  ```
- Confirm the deploy landed: `curl https://worship.hesed.sbs/api/version` — the
  `buildTag` (content hash of every file under `./public`) must change.
- Homelab node is NOT always reachable on LAN; SSH via the VPS hop:
  `ssh root@193.160.119.172` → `ssh root@10.8.0.2` (WireGuard). Container name:
  `ytplayer-main-1dihzn-ytplayer-1`. DB: libsql file `/app/data/ytplayer.db`
  (query with `docker exec <c> bun -e '...' `using `@libsql/client`).

## Update-flow architecture (fragile — read before touching)
- `GET /sw.js` is served by the server with the real `BUILD_TAG` **injected by
  regex** over the fallback expression in `frontend/sw.js`. Never switch back to
  an exact-string replace: when the fallback literal was bumped (`v1.0.3`→`v1.0.4`)
  the exact match silently failed, the SW version froze, and **no client ever
  received another update** while `/api/version` kept announcing one — the
  "Update available keeps showing" bug.
- `BUILD_TAG` hashes **every** file under `./public` recursively. Don't reduce it
  to a file subset; a change to an unlisted shell file would stop busting caches.
- **The banner has ONE rule** (`maybeShowUpdateBanner` in app.js): show it only
  when the build this page runs differs from `/api/version`. The server stamps
  the running build into index.html (`<meta name="ytp-build">`, at request
  time like sw.js's BUILD_TAG); the SW caches that index.html with the rest of the
  shell, so the meta always describes the code in the tab. The poll, a waiting
  worker and the SW_UPDATE_AVAILABLE broadcast are only *prompts to re-check*.
  This loop was "fixed" three times by chasing individual triggers — don't add
  a trigger that calls `showUpdateBanner()` directly.
- **Background download + instant swap (added 2026-09-22).** A new worker
  precaches the whole shell during `install`, so by the time the user presses
  "Refresh UI" the build is usually already on disk. `applyUpdate` asks the
  **waiting** worker `CACHE_STATUS` over a `MessageChannel`; when it answers
  `ready: true` (it re-checks every SHELL url, since a cache can be evicted)
  the download is skipped entirely and the swap is immediate.
  Two rules keep this safe: the probe is **opt-in** (`askStatus`) — never
  implicit, because a probe awaiting a reply hangs forever if the caller
  injects a `setTimeout` that never fires (the existing tests do exactly
  that) — and *any* doubt (no reply, timeout, thrown error, `ready: false`)
  falls back to the all-or-nothing download below, which is still what
  guarantees correctness. `maybeShowUpdateBanner` also calls `prefetchUpdate()`
  → `registration.update()` so the download starts the moment a new build is
  seen rather than when the user clicks.
- **Refresh UI** (`frontend/sw-update.js`): `refreshShellInPlace()` re-downloads
  every file the shell caches hold — cache-busted (`?__ytpfresh=`) so even an
  OLD worker's cache-first handler can't answer from its cache, 3 tries per file,
  all-or-nothing — and writes them into every versioned shell cache; then it
  activates a waiting worker (SKIP_WAITING) if any and reloads once. A failed
  download shows an error toast and re-offers later; it never reloads into the
  old shell. `checkUpdateOutcome()` verifies after the reload (sessionStorage,
  max 3 attempts) instead of looping.
- **Why it kept looping on prod:** the VPS→homelab link is slow and drops
  requests. The SW install (`cache.addAll`, all-or-nothing) failed, Refresh UI
  reloaded into the old cached shell, and the partial `ytplayer-<tag>` cache
  left by the failed install later made activate broadcast "update available"
  to pages that were already current. sw.js now precaches with
  `cache: 'reload'`, retries each file, and deletes its partial cache on failure.
- Reproduce with the throttling/stalling proxy approach: WebKit (Playwright on
  the Windows side) or Chrome over CDP against a scratch copy of the server,
  stalling every Nth shell request after "deploying" v2. A fast local link never
  shows the bug.

## Cache layout & offline thumbnails
- Three cache families, and the split matters on activate: the **versioned shell
  cache** `ytplayer-<BUILD_TAG>` is evicted on every deploy, while the **utility
  caches** `ytplayer-thumbs` and `ytplayer-fonts` are listed in `UTILITY_CACHES`
  and deliberately survive it. Adding a new utility cache means adding it there
  too, or it gets wiped on the next deploy.
- **Thumbnails are cache-first, not stale-while-revalidate** — a given thumbnail
  URL is immutable, so revalidating just burns a round trip per image per launch.
- **The opaque-response trap (this silently emptied the thumb cache for months).**
  An `<img>` to another origin is a **no-cors** request, so `fetch(request)` in the
  SW resolves to an **opaque** response with `status === 0` — not 200. The old
  guard was `if (r.status === 200) cache.put(...)`, which rejected every single
  thumbnail, so `ytplayer-thumbs` was permanently empty and offline showed broken
  images (measured on prod: 0 entries after browsing pages full of visible thumbs).
  Fix in `thumbnail()`: re-issue the request in `cors` mode — ytimg/ggpht all send
  `Access-Control-Allow-Origin: *` — and cache that readable response; an opaque
  one is accepted only as a last resort. **Never reintroduce a bare `status === 200`
  check on a cross-origin subresource.**
- Thumbnail hosts live in `THUMB_HOSTS`. An unlisted host doesn't error — it just
  bypasses the cache and breaks offline, so add mirrors/avatar hosts there.
- The thumb cache is capped at `THUMB_CACHE_MAX` (800, oldest-first via the
  insertion-ordered `cache.keys()`). Keep a cap: CacheStorage and the OPFS offline
  audio share one origin quota, and opaque entries are padded to ~7 MB each for
  quota accounting, so an unbounded thumb cache can evict saved audio.
- App side (`app.js`): `warmThumb()` pulls artwork through the SW when a video is
  saved offline, and `warmOfflineThumbs()` runs a bounded launch backfill
  (`THUMB_WARM_MAX` = 400, 4 at a time) over cached ids + playlist videos so
  libraries saved before this fix repair themselves. Thumbnails only cache when
  something requests them — a device needs one online launch to get offline art.
- `staleWhileRevalidate()` (fonts) must not `return cached || networkFetch` bare:
  the fetch resolves to `null` offline and `respondWith(null)` throws. It ends
  with `|| Response.error()`.

## Section rail + foldable cards (phones)
`SectionRail` in app.js draws a floating right-hand rail on phones with one
icon per visible card (player · now playing · lyrics · related · list). It
follows the scroll, a tap jumps to that card, and a tap on the card you are
**already** on folds/unfolds it (`data.settings.foldedCards`, per device).
Folding hides a card's body but never its header, so it stays a landmark you
can scroll to and reopen; the Related card's own +/− button routes through
`SectionRail.setFolded` so the two can never disagree.

Two things about this page that break naive implementations:
- **The document does not scroll.** `.player-pane` is the scroll container in
  the browser, but the installed PWA in portrait scrolls `.body` instead
  (`.player-pane` becomes `overflow-y: visible` there). `scrollerOf()` walks up
  to whichever ancestor actually scrolls, and `viewBox()` measures against it.
- **Scroll events do not bubble**, so a `window` scroll listener never fires
  for an element scroller — the listener is on `document` with `capture: true`.
Cards also appear outside `render()` (Related arrives with the video's related
list), so `onScroll` re-runs `build()`, which is a no-op unless the visible set
changed; without that the rail could highlight nothing at all.

## First paint vs. the network (launch)
`boot()` used to `await` the profile pull and the shared-playlist inbox before
the first `render()`, so on a slow link the sidebar stayed empty for as long as
the network took. Everything the device knows is already in localStorage, so it
now paints immediately and reconciles afterwards:
- `?list=` / `?profile=` share links still run **before** the first paint — they
  *replace* the synced slice, so painting first would flash the old playlists
  and swap them out. They are skipped entirely when the parameter is absent.
- `syncOnLaunch()` then runs the network pass with a spinner (`#syncSpinner`,
  beside the Playlists header, `setSyncing()` is counted so the last finisher
  clears it) and re-renders **only if** `playlistFingerprint()` changed — a
  needless render would drop the sidebar's scroll position.

## Data model quirks
- Client state persists in localStorage key **`_ytpdata`** and syncs (debounced
  400 ms) to `POST /api/user/sync`, keyed by a browser fingerprint.
- **A-B loop markers are per-song-per-playlist**: stored on the playlist's own
  copy of the video (`entry.ab = {a, b}`) when playback source is that playlist;
  `data.abMarkers[videoId]` is only the fallback for non-playlist playback.
- **Online profiles** (`profiles` table, `/api/profile/*`): named cross-device
  sync where the lowercase profile NAME is the only credential (passkey-style,
  by design). Client stores `data.profile = {name, syncedAt}`; sync is
  last-write-wins — push debounced on every persist(), pull on app launch when
  the server's `updated_at` is newer than the local `syncedAt`.
- **Profile share links**: `?profile=<name>` is consumed by `adoptProfileFromUrl()`
  at boot, before any rendering. It strips the param via `replaceState` (so a
  reload can't re-fire it) and confirms first when the device already has
  playlists/history or another profile — adopting *replaces* the synced slice.
  The name comes off the URL untrusted, hence `escapeHtml()` on it.
- **Empty-home playlist grid**: `renderHomePlaylists()` swaps the branding hero in
  `#playerPlaceholder` for the user's playlists (hero is the no-playlists
  fallback). It is driven from the tail of `renderSidebar()` — not from
  `render()` — so every playlist mutation refreshes both in one place.

## Testing
- Unit: **`node --test frontend/*.test.js`** (sw, sw-update, async-guard, video-edit, lyrics-core, stats-core).
  Server: **`cd server && bun run test`** — runs each file in its own process (db.js is a
  singleton, so two test files in one `bun test` run share one temp DB; bare `bun test`
  also drags in the frontend node tests via the `public` symlink). media-cache needs a
  60 s hook timeout for its ffmpeg fixtures.
  Local e2e needs a CURRENT yt-dlp — a 2-month-old one 403s on every download.
  The directory form `node --test frontend/` fails on Node 22 with
  `Cannot find module .../frontend` — it resolves the dir as a module, not a test
  glob. That's the harness, not the tests.
- E2E: `npx playwright test` — WebKit iPhone-12 profile against a static serve of
  `frontend/` (needs `npx playwright install webkit`). A spurious update banner
  will make the settings-panel specs fail with `#modal intercepts pointer events`
  — that failure mode is a real app bug, not test flake. On the WSL laptop the
  WebKit install fails host validation (missing `libgtk-4`, `libgstreamer*`, …,
  needs `sudo npx playwright install-deps`); chromium is already downloaded, so
  ad-hoc rendering/offline checks can drive it directly instead.
- Service-worker behaviour (thumb caching, offline) is only provable in a real
  browser: serve `frontend/` statically, let the SW take control (needs a second
  reload), then `context.setOffline(true)` and assert `img.naturalWidth > 0` plus
  the `ytplayer-thumbs` entry count. Asserting against the browser's own HTTP
  cache proves nothing — check the Cache API entry count.
- Test records on prod use `Probe */Recon *` names; clean via the container DB,
  children (`video_history`) first.

## Harness

Skills live in `.agents/skills/` (symlinked into `.claude/skills/`):
- **deploy-prod** — the Dokploy deploy + buildTag verification flow (manual-only; use for any "deploy"/"release" request).
- **mobile-app-ui-design** — UI/UX design work on the PWA screens.
- **lyrics-lookup** — fill in songs that have NO lyrics (LRCLIB → agy web search → local faster-whisper).
- **lyrics-regenerate** — back up, then replace wrong/mistimed lyrics from LRCLIB (`scripts/lyrics/lrclib_regen.py`).
- **lyrics-agy** — last resort when LRCLIB has nothing: agy (flat-rate) searches the web and often returns
  LRC-timed lyrics (`scripts/lyrics/agy_lyrics.py`). Also the `--from-file` path for pasting lyrics you already have.
  agy is NOT deterministic — the script asks up to `--tries` times and keeps the best (timed > untimed > longer).
  Its answer arrives on **stderr**, long answers are truncated behind a `keep_id`, and a quota-failed instance
  returns error prose inside a `STATUS: ok` envelope that will parse as lyrics if you let it.

### Matching a song on LRCLIB (learned the hard way)
`/api/streams` gives the YouTube **channel**, not the artist, so a title+artist
query finds nothing for a lyric-video channel ("Integrity Worship", "Christian
Lyrics") — `lrclib_lookup` falls back to a title-only search and then ranks
candidates whose artist *verifies* 50 points above those that don't. Two guards
sit on the result, and both exist because they caught a real wrong song:
- `artist_ok()` ignores `GENERIC` words. Channel "**Christian** Lyrics" once
  vouched for a track featuring **Christian** Burns (Nicky Romero, "Still the
  Same Man"). Never verify an artist on a word that says nothing about who
  recorded the song.
- `title_run()` compares **whole words, never substrings** — "Still" is a
  substring of "(You Can Still) Rock in America", and a one-word title is
  rejected outright. It gates the `LENGTH` rule (same title, within 3 s of the
  same length ⇒ accept even when the artist can't be checked).
An unverified match is printed as `UNSURE` and skipped; `--loose` overrides that
and is almost always the wrong answer.

### Admin lyric editor (`frontend/admin.html`)
`/admin?v=<id>` (or the Edit/Lyrics buttons) opens a song with its audio from
`/api/streams`, the waveform from `/api/media/:id/peaks`, and one row per line:
**Set** stamps the playhead, ±0.2 s nudges, **Tap mode** stamps the highlighted
line on Space and walks down, plus shift-all, line/section/cue kinds, a ✎ Text
LRC view and a ⤓ LRCLIB pull. Saves are optimistic (`baseRev`, 409 → keep
mine / load theirs). **Rows are built once per load and then only their classes
and values change** — re-rendering on every tick would steal focus from the
field being typed in and reset the scroll position.

## Commit rules
One changeset = one commit, single-line imperative message, **no AI attribution
of any kind** (global rule). `git push origin main` pushes to both remotes.