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/player.ts

@@ -1,8 +1,8 @@
 import { createSlice, type PayloadAction } from '@reduxjs/toolkit';
 
-type RepeatMode = 'none' | 'one' | 'all';
+export type RepeatMode = 'none' | 'one' | 'all';
 
-interface PlayerState {
+export interface PlayerState {
   currentTrackId: string | null;
   isPlaying: boolean;
   position: number;
@@ -15,7 +15,7 @@ interface PlayerState {
   isQueueOpen: boolean;
 }
 
-const initialState: PlayerState = {
+export const playerInitialState: PlayerState = {
   currentTrackId: null,
   isPlaying: false,
   position: 0,
@@ -25,14 +25,12 @@ const initialState: PlayerState = {
   repeat: 'none',
   shuffle: false,
   isNowPlayingOpen: false,
-  // STUB: open by default so the queue drawer look is visible before a backend
-  // exists (pairs with DEMO_QUEUE). Default to false once real playback lands.
-  isQueueOpen: true,
+  isQueueOpen: false,
 };
 
 export const playerSlice = createSlice({
   name: 'player',
-  initialState,
+  initialState: playerInitialState,
   reducers: {
     play(state, action: PayloadAction<string>) {
       state.currentTrackId = action.payload;