feat(offline): make the web UI usable without a reachable backend

Three tiers of offline support, all scoped to the active backend's
localStorage namespace (mirroring the auth slice):

Tier 1 — persist client state. queue + player slices are saved (queue
entries/index/source; player track/position/volume/repeat/shuffle) and
rehydrated on load, so a reload with no backend restores where the user
left off. Playback never auto-resumes (browsers block autoplay). Retires
the DEMO_QUEUE and isQueueOpen:true stubs.

Tier 2 — persist the RTK Query cache. Last-seen library/albums/artists
are snapshotted (fulfilled queries only) and replayed via RTKQ's
extractRehydrationInfo at startup, so the library renders read-only when
the backend is down. ConnectionStatus tooltip flags cached data offline.
No server data is copied into a slice — the cache feeds itself back.

Tier 3 — service worker audio + cover cache (PWA). Audio streams are
cached keyed by content id (token stripped), range-aware (synthetic 206
slicing), with a 500MB LRU cap, so already-played tracks play fully
offline. Cover art uses stale-while-revalidate in its own bounded cache.
Module worker (ESM); pure helpers split into sw-core.js and unit-tested.
Web app manifest enables "Install app". Player source badge now reflects
real cached state.

tsc clean, lint clean, 19 new tests pass, production build verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

rsbuild.config.ts

@@ -32,5 +32,16 @@ export default defineConfig({
   // no manual source.define needed. See src/config/env.ts.
   html: {
     title: 'MCMA',
+    // PWA: link the manifest + declare theme/icon so the browser offers
+    // "Install app". The service worker (audio offline cache) is registered
+    // from src/index.tsx, not here.
+    tags: [
+      {
+        tag: 'link',
+        attrs: { rel: 'manifest', href: '/manifest.webmanifest' },
+      },
+      { tag: 'meta', attrs: { name: 'theme-color', content: '#0b0b0b' } },
+      { tag: 'link', attrs: { rel: 'apple-touch-icon', href: '/favicon.png' } },
+    ],
   },
 });