feat: add passkey-style online profiles for cross-device sync and show build time in About

CLAUDE.md

@@ -46,6 +46,11 @@ The PWA is what runs in production; `legacy/` holds the old native-only docs.
 - **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`.
 
 ## Testing
 - Unit: `node --test frontend/` (sw, sw-update, async-guard).