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/App.tsx

@@ -54,6 +54,7 @@ import { version } from '../package.json';
 import { useConnectionStatus } from './hooks/useConnectionStatus';
 import { useAuthStore } from './store/authStore';
 import { useOfflineStore } from './store/offlineStore';
+import { initHotCachePrefetch } from './hotCachePrefetch';
 import { usePlayerStore, initAudioListeners } from './store/playerStore';
 import { useThemeStore } from './store/themeStore';
 import { useFontStore } from './store/fontStore';
@@ -461,6 +462,10 @@ export default function App() {
     return initAudioListeners();
   }, []);
 
+  useEffect(() => {
+    return initHotCachePrefetch();
+  }, []);
+
   useEffect(() => {
     useGlobalShortcutsStore.getState().registerAll();
   }, []);