feat: add hot playback cache (queue prefetch, alpha)

Ephemeral on-disk cache for upcoming queue tracks. Adds Rust commands (download/delete/purge), hotCacheStore with LRU eviction, serial prefetch worker, playback gate, and Settings UI. Disabled by default.

- fix: boundary check before file delete in delete_hot_cache_track
- fix: remove stale languageRu2 key from ru.ts
- fix: restore accidentally removed lyricsServerFirst/fsLyricsToggle/lyricsSource* strings in ru.ts

src/hotCachePrefetch.ts

@@ -0,0 +1,184 @@
+import { invoke } from '@tauri-apps/api/core';
+import { buildStreamUrl } from './api/subsonic';
+import { useAuthStore } from './store/authStore';
+import { useHotCacheStore } from './store/hotCacheStore';
+import { useOfflineStore } from './store/offlineStore';
+import { usePlayerStore } from './store/playerStore';
+import { getDeferHotCachePrefetch } from './utils/hotCacheGate';
+
+const PREFETCH_AHEAD = 5;
+
+type PrefetchJob = { trackId: string; serverId: string; suffix: string };
+
+let debounceTimer: ReturnType<typeof setTimeout> | null = null;
+const pendingQueue: PrefetchJob[] = [];
+let workerRunning = false;
+
+function debounceMs(): number {
+  const s = useAuthStore.getState().hotCacheDebounceSec;
+  if (!Number.isFinite(s) || s < 0) return 0;
+  return Math.min(600, s) * 1000;
+}
+
+function enqueueJobs(jobs: PrefetchJob[]) {
+  const seen = new Set(pendingQueue.map(j => `${j.serverId}:${j.trackId}`));
+  for (const j of jobs) {
+    const k = `${j.serverId}:${j.trackId}`;
+    if (seen.has(k)) continue;
+    seen.add(k);
+    pendingQueue.push(j);
+  }
+  void runWorker();
+}
+
+async function runWorker() {
+  if (workerRunning) return;
+  workerRunning = true;
+  try {
+    while (pendingQueue.length > 0) {
+      const auth = useAuthStore.getState();
+      if (!auth.isLoggedIn || !auth.hotCacheEnabled || !auth.activeServerId) {
+        pendingQueue.length = 0;
+        break;
+      }
+
+      while (getDeferHotCachePrefetch()) {
+        await new Promise(r => setTimeout(r, 150));
+      }
+
+      const job = pendingQueue.shift();
+      if (!job) break;
+
+      const maxBytes = Math.max(0, auth.hotCacheMaxMb) * 1024 * 1024;
+      if (maxBytes <= 0) continue;
+
+      const offline = useOfflineStore.getState();
+      if (offline.isDownloaded(job.trackId, job.serverId)) continue;
+      if (useHotCacheStore.getState().entries[entryKey(job.serverId, job.trackId)]) continue;
+
+      const { queue, queueIndex } = usePlayerStore.getState();
+      const wantIds = new Set(
+        queue
+          .slice(queueIndex + 1, queueIndex + 1 + PREFETCH_AHEAD)
+          .map(t => t.id),
+      );
+      if (!wantIds.has(job.trackId)) continue;
+
+      const url = buildStreamUrl(job.trackId);
+      try {
+        const customDir = auth.hotCacheDownloadDir || null;
+        const res = await invoke<{ path: string; size: number }>('download_track_hot_cache', {
+          trackId: job.trackId,
+          serverId: job.serverId,
+          url,
+          suffix: job.suffix,
+          customDir,
+        });
+        useHotCacheStore.getState().setEntry(job.trackId, job.serverId, res.path, res.size);
+        const fresh = usePlayerStore.getState();
+        await useHotCacheStore.getState().evictToFit(
+          fresh.queue,
+          fresh.queueIndex,
+          maxBytes,
+          auth.activeServerId,
+          customDir,
+        );
+      } catch {
+        /* network / HTTP — skip */
+      }
+    }
+  } finally {
+    workerRunning = false;
+    if (pendingQueue.length > 0) void runWorker();
+  }
+}
+
+function entryKey(serverId: string, trackId: string): string {
+  return `${serverId}:${trackId}`;
+}
+
+function scheduleReplan() {
+  const auth = useAuthStore.getState();
+  if (!auth.isLoggedIn || !auth.hotCacheEnabled || !auth.activeServerId) {
+    if (debounceTimer) {
+      clearTimeout(debounceTimer);
+      debounceTimer = null;
+    }
+    return;
+  }
+
+  if (debounceTimer) clearTimeout(debounceTimer);
+  const ms = debounceMs();
+  debounceTimer = setTimeout(() => {
+    debounceTimer = null;
+    void replanNow();
+  }, ms);
+}
+
+async function replanNow() {
+  const auth = useAuthStore.getState();
+  if (!auth.isLoggedIn || !auth.hotCacheEnabled || !auth.activeServerId) return;
+
+  const serverId = auth.activeServerId;
+  const maxBytes = Math.max(0, auth.hotCacheMaxMb) * 1024 * 1024;
+  const customDir = auth.hotCacheDownloadDir || null;
+  if (maxBytes <= 0) return;
+
+  const { queue, queueIndex, currentRadio } = usePlayerStore.getState();
+  if (currentRadio) return;
+
+  const offline = useOfflineStore.getState();
+  const hot = useHotCacheStore.getState();
+
+  await hot.evictToFit(queue, queueIndex, maxBytes, serverId, customDir);
+
+  const targets = queue.slice(queueIndex + 1, queueIndex + 1 + PREFETCH_AHEAD);
+  const jobs: PrefetchJob[] = [];
+  for (const t of targets) {
+    if (offline.isDownloaded(t.id, serverId)) continue;
+    if (hot.entries[entryKey(serverId, t.id)]) continue;
+    jobs.push({
+      trackId: t.id,
+      serverId,
+      suffix: t.suffix || 'mp3',
+    });
+  }
+  enqueueJobs(jobs);
+}
+
+/**
+ * Subscribe to queue/auth changes and run debounced prefetch.
+ * Call once from the app shell.
+ */
+export function initHotCachePrefetch(): () => void {
+  let lastQueueRef: unknown = null;
+  let lastQueueIndex = -1;
+  const unsubPlayer = usePlayerStore.subscribe(state => {
+    const q = state.queue;
+    const i = state.queueIndex;
+    if (q === lastQueueRef && i === lastQueueIndex) return;
+    const onlyIndexMoved = q === lastQueueRef && i !== lastQueueIndex;
+    lastQueueRef = q;
+    lastQueueIndex = i;
+    if (onlyIndexMoved) void replanNow();
+    else scheduleReplan();
+  });
+
+  let lastAuthSig = '';
+  const unsubAuth = useAuthStore.subscribe(state => {
+    const sig = `${state.hotCacheEnabled}:${state.hotCacheDebounceSec}:${state.hotCacheMaxMb}:${state.hotCacheDownloadDir ?? ''}:${state.activeServerId ?? ''}:${state.isLoggedIn}`;
+    if (sig === lastAuthSig) return;
+    lastAuthSig = sig;
+    if (state.hotCacheEnabled && state.isLoggedIn) scheduleReplan();
+  });
+
+  void replanNow();
+
+  return () => {
+    unsubPlayer();
+    unsubAuth();
+    if (debounceTimer) clearTimeout(debounceTimer);
+    debounceTimer = null;
+    pendingQueue.length = 0;
+  };
+}