Share target: shared YouTube links play, shared audio/video uploads to the user's library; page shortcuts

- manifest share_target (POST multipart: title/text/url + audio/video files)
  and eight shortcuts (Search, Queue, Saved, Downloads, History, Library,
  Notes, Settings). /?view=<page> now actually opens that page — the old
  shortcuts pointed at it but nothing read it.
- sw.js receives the share: files are parked in the ytp-share-inbox cache (not
  a versioned shell cache, so deploys never evict it) and the app is opened;
  links/text go to /?shared=. A server POST /share-target fallback covers the
  first visit before a worker is in control (links only).
- The app plays a YouTube link found anywhere in the shared text (watch,
  youtu.be, shorts, live, embed, music; keeps t=), otherwise searches the text.
  /?shared=<link> also works from an iOS Shortcut (iOS has no share target).
- Shared files upload with progress to PUT /api/uploads/shared and join a
  "Shared uploads" playlist; network failures stay in the inbox for the next
  open, refusals are dropped with the reason.
- The public route is bounded: audio/video only (ffprobe-validated, error text
  without server paths), 500 MB per file, 2 GB and 20 a day per device, 50 GB
  for all shared uploads, one at a time per device (all env-tunable). Shared
  uploads are unlisted: reachable by id, never in anyone else's search.
  uploads gains owner + listed columns (idempotent ALTER).

frontend/manifest.webmanifest

@@ -8,7 +8,10 @@
   "scope": "/",
   "theme_color": "#1a1311",
   "background_color": "#1a1311",
-  "categories": ["music", "entertainment"],
+  "categories": [
+    "music",
+    "entertainment"
+  ],
   "icons": [
     {
       "src": "/icons/icon-192.png",
@@ -29,13 +32,135 @@
       "name": "Search",
       "short_name": "Search",
       "url": "/?view=search",
-      "description": "Search YouTube"
+      "description": "Search YouTube",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Queue",
+      "short_name": "Queue",
+      "url": "/?view=queue",
+      "description": "What plays next",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Saved",
+      "short_name": "Saved",
+      "url": "/?view=saved",
+      "description": "Videos saved for offline",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Downloads",
+      "short_name": "Downloads",
+      "url": "/?view=downloads",
+      "description": "Saves in progress and paused",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
     },
     {
       "name": "History",
       "short_name": "History",
       "url": "/?view=history",
-      "description": "Recently watched videos"
+      "description": "Recently watched videos",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Library",
+      "short_name": "Library",
+      "url": "/?view=library",
+      "description": "Your playlists",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Lyrics & notes",
+      "short_name": "Notes",
+      "url": "/?view=notes",
+      "description": "Lyrics, chapters and notes",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
+    },
+    {
+      "name": "Settings",
+      "short_name": "Settings",
+      "url": "/?view=settings",
+      "description": "App settings",
+      "icons": [
+        {
+          "src": "/icons/icon-192.png",
+          "sizes": "192x192",
+          "type": "image/png"
+        }
+      ]
     }
-  ]
+  ],
+  "share_target": {
+    "action": "/share-target",
+    "method": "POST",
+    "enctype": "multipart/form-data",
+    "params": {
+      "title": "title",
+      "text": "text",
+      "url": "url",
+      "files": [
+        {
+          "name": "media",
+          "accept": [
+            "audio/*",
+            "video/*",
+            ".mp3",
+            ".m4a",
+            ".aac",
+            ".wav",
+            ".flac",
+            ".ogg",
+            ".opus",
+            ".mp4",
+            ".mov",
+            ".webm",
+            ".mkv"
+          ]
+        }
+      ]
+    }
+  }
 }