feat: PWA Phase 1 — offline audio download and playback

- API: GET /versions/:id/audio?quality=stream|original (server proxy for SW caching)
- API: GET /versions/:id/waveform-data (server proxy for offline waveform)
- SW: cache-first from musichub-offline-v1 for proxied audio/waveform endpoints
- Client: IDB-backed offline store (idb lib) with progress-tracked download
- UI: per-version offline download button with stream/original quality picker
- UI: /offline page with storage estimate and remove-all action
- Manifest: shortcuts for Dashboard + Offline-Tracks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/static/manifest.webmanifest

@@ -10,6 +10,20 @@
   "theme_color": "#f43f5e",
   "lang": "de",
   "categories": ["music", "productivity"],
+  "shortcuts": [
+    {
+      "name": "Offline-Tracks",
+      "short_name": "Offline",
+      "url": "/offline",
+      "icons": [{ "src": "/icon-192.png", "sizes": "192x192" }]
+    },
+    {
+      "name": "Dashboard",
+      "short_name": "Dashboard",
+      "url": "/dashboard",
+      "icons": [{ "src": "/icon-192.png", "sizes": "192x192" }]
+    }
+  ],
   "icons": [
     {
       "src": "/icon-192.png",