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/package.json

@@ -14,6 +14,7 @@
     "@fontsource-variable/inter": "^5.2.8",
     "@music-hub/shared": "workspace:*",
     "@sveltejs/adapter-node": "^5.5.4",
+    "idb": "^8.0.3",
     "wavesurfer.js": "^7.12.5"
   },
   "devDependencies": {