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>

bun.lock

@@ -32,6 +32,7 @@
         "@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": {
@@ -487,6 +488,8 @@
 
     "hono": ["hono@4.12.9", "", {}, "sha512-wy3T8Zm2bsEvxKZM5w21VdHDDcwVS1yUFFY6i8UobSsKfFceT7TOwhbhfKsDyx7tYQlmRM5FLpIuYvNFyjctiA=="],
 
+    "idb": ["idb@8.0.3", "", {}, "sha512-LtwtVyVYO5BqRvcsKuB2iUMnHwPVByPCXFXOpuU96IZPPoPN6xjOGxZQ74pgSVVLQWtUOYgyeL4GE98BY5D3wg=="],
+
     "is-core-module": ["is-core-module@2.16.1", "", { "dependencies": { "hasown": "^2.0.2" } }, "sha512-UfoeMA6fIJ8wTYFEUjelnaGI67v6+N7qXJEvQuIGa99l4xsCruSYOVSQ0uPANn4dAzm8lkYPaKLrrijLq7x23w=="],
 
     "is-module": ["is-module@1.0.0", "", {}, "sha512-51ypPSPCoTEIN9dy5Oy+h4pShgJmPCygKfyRCISBI+JoWT/2oJvK8QPxmwv7b/p239jXrm9M1mlQbyKJ5A152g=="],