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/store/slices/queue.ts

@@ -1,6 +1,6 @@
 import { createSlice, type PayloadAction } from '@reduxjs/toolkit';
 
-type QueueSource =
+export type QueueSource =
   | 'manual'
   | 'album'
   | 'playlist'
@@ -8,7 +8,7 @@ type QueueSource =
   | 'search'
   | 'radio';
 
-interface QueueEntry {
+export interface QueueEntry {
   trackId: string;
   title: string;
   artistName: string;
@@ -17,7 +17,7 @@ interface QueueEntry {
   albumArtUrl?: string;
 }
 
-interface QueueState {
+export interface QueueState {
   entries: QueueEntry[];
   currentIndex: number;
   source: QueueSource;
@@ -25,52 +25,17 @@ interface QueueState {
   sourceName: string | null;
 }
 
-// STUB demo queue — purely client-side display data so the player bar and
-// queue drawer render with content before the backend exists. Delete this
-// block (reset entries/currentIndex/source to the empty values) once real
-// playback wires tracks into the queue.
-const DEMO_QUEUE: QueueEntry[] = [
-  {
-    trackId: 'd1',
-    title: 'Quiet Storage',
-    artistName: 'Cyan Atlas',
-    albumTitle: 'Night Index',
-    durationMs: 312_000,
-  },
-  {
-    trackId: 'd2',
-    title: 'Magnetic North',
-    artistName: 'Tidal Bloom',
-    albumTitle: 'Ferric Coast',
-    durationMs: 243_000,
-  },
-  {
-    trackId: 'd3',
-    title: 'Ambergris',
-    artistName: 'Møller',
-    albumTitle: 'Warm Static',
-    durationMs: 201_000,
-  },
-  {
-    trackId: 'd4',
-    title: 'Slow Carrier',
-    artistName: 'Tidal Bloom',
-    albumTitle: 'Ferric Coast',
-    durationMs: 301_000,
-  },
-];
-
-const initialState: QueueState = {
-  entries: DEMO_QUEUE,
-  currentIndex: 0,
-  source: 'radio',
+export const queueInitialState: QueueState = {
+  entries: [],
+  currentIndex: -1,
+  source: 'manual',
   sourceId: null,
-  sourceName: 'My radio',
+  sourceName: null,
 };
 
 export const queueSlice = createSlice({
   name: 'queue',
-  initialState,
+  initialState: queueInitialState,
   reducers: {
     setQueue(
       state,