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>

public/manifest.webmanifest

@@ -0,0 +1,19 @@
+{
+  "name": "MCMA — Music",
+  "short_name": "MCMA",
+  "description": "Self-hosted music — control center and offline-capable player.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#0b0b0b",
+  "theme_color": "#0b0b0b",
+  "icons": [
+    {
+      "src": "/favicon.png",
+      "sizes": "128x128",
+      "type": "image/png",
+      "purpose": "any"
+    }
+  ]
+}