feat: wire up WEB-mode OPFS offline cache for PWA

The PWA conversion shipped sw.js, opfs.js, manifest and icons, but app.js
was never updated to use them — it still routed cache_* calls through the
native bridge (call('cache.list', ...)), which throws 'No native bridge
available' in a browser. That surfaced as 'Offline cache unavailable /
not available in this build' on the Saved videos and Settings pages.

- Add WEB mode detection (no Tauri/zero bridge present)
- Route cache_* through OPFS (opfsDownload/Status/List/Delete/Clear)
- Proxy downloads via /api/download so the browser never hits YouTube CDN
- Load fingerprint.js + opfs.js; add manifest link and PWA meta tags
- Relax CSP to allow blob: media/img and worker-src 'self' for the SW

package.json

@@ -6,6 +6,7 @@
     "setup": "node scripts/setup-ytdlp.js",
     "update-ytdlp": "node scripts/setup-ytdlp.js --force",
     "make-icon": "node scripts/make-icon.js",
+    "make-pwa-icons": "node scripts/make-pwa-icons.js",
     "tauri": "tauri",
     "tauri:dev": "tauri dev",
     "tauri:build": "tauri build",