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>

src/index.tsx

@@ -10,6 +10,7 @@ import { BrowserRouter } from 'react-router';
 import { ThemeProvider, TooltipProvider } from '@olly/modern-sk';
 import { store } from './store';
 import { AppRoutes } from './routes';
+import { registerServiceWorker } from './lib/sw';
 
 // Import all endpoint injections to ensure they are registered
 import './api/endpoints/auth';
@@ -21,6 +22,10 @@ import './api/endpoints/storage';
 import './api/endpoints/admin';
 import './api/endpoints/upload';
 
+// Tier 3 offline: register the audio-caching service worker (no-op if the
+// browser/origin doesn't support it).
+registerServiceWorker();
+
 const rootEl = document.getElementById('root');
 if (rootEl) {
   // grained black-ish background + base text color from modern-sk