refactor(queue): track resolver + selectors (thin-state phase 2) (#859)

* refactor(queue): add queue track resolver (thin-state phase 2a)

Standalone resolver: QueueItemRef → Track via index batch
(library_get_tracks_batch, ≤100/call) → network getSong fallback (P8), into a
bounded LRU cache. Holds raw tracks; session star/rating overrides (F4) merged
on read via applyQueueOverrides. Sync getCachedTrack + subscribeQueueResolver
for selectors; resolveVisibleRange prefetches a [-50, +200] window; carries
queue-only flags from refs; invalidate drops entries after a sync succeeds.

Not wired into the store/UI yet — phase 2b does that.

* refactor(queue): extract toQueueItemRefs helper

Pure helper deriving thin QueueItemRefs from a Track[] queue (per-item
serverId, queue-only flags), shared by the persist partialize and the
upcoming phase-2b resolver bridge. No behaviour change.

* refactor(queue): resolver bridge + queue selectors (thin-state phase 2b)

Seed the resolver cache from the canonical queue (queueResolverBridge, a
windowed [-50, +200] seed around the current index) and add the stable queue
selectors (useQueueTrackAt / useCurrentTrack / useQueueItems).

Additive: the store stays queue: Track[]-canonical; consumers migrate onto the
selectors in phase 3, and the selector impls move to the resolver once
queue: Track[] is dropped in phase 4. No mutation or persist change — the
persisted queueItems keeps its single restore role (no dual-role clash).

* docs(changelog): on-demand queue track loading groundwork (#859)

src/hooks/useQueueTracks.ts

@@ -0,0 +1,28 @@
+import { useMemo } from 'react';
+import { usePlayerStore } from '../store/playerStore';
+import type { QueueItemRef, Track } from '../store/playerStoreTypes';
+import { toQueueItemRefs } from '../utils/library/queueItemRef';
+
+/**
+ * Stable queue selectors (queue thin-state). Consumers migrate onto these in
+ * phase 3. Today they read the canonical `queue: Track[]`; once it's dropped
+ * (phase 4) the implementations move to the resolver (`queueTrackResolver`)
+ * without changing these signatures.
+ */
+
+/** The track at a queue index, or null. */
+export function useQueueTrackAt(idx: number): Track | null {
+  return usePlayerStore(s => s.queue[idx] ?? null);
+}
+
+/** The currently playing track, or null. */
+export function useCurrentTrack(): Track | null {
+  return usePlayerStore(s => s.currentTrack);
+}
+
+/** The whole queue as thin refs (derived; memoized on queue/server identity). */
+export function useQueueItems(): QueueItemRef[] {
+  const queue = usePlayerStore(s => s.queue);
+  const serverId = usePlayerStore(s => s.queueServerId);
+  return useMemo(() => toQueueItemRefs(serverId ?? '', queue), [serverId, queue]);
+}