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/locales/en.ts

@@ -404,6 +404,8 @@ export const enTranslation = {
     cacheDesc: 'Cover art and artist images. When full, the oldest entries are removed automatically. Offline albums are not auto-removed, but will be deleted when clearing the cache manually.',
     cacheUsedImages: 'Images:',
     cacheUsedOffline: 'Offline tracks:',
+    cacheUsedHot: 'Size on disk:',
+    hotCacheTrackCount: 'Tracks in cache:',
     cacheMaxLabel: 'Max. size',
     cacheClearBtn: 'Clear Cache',
     cacheClearWarning: 'This will also remove all offline albums from the library.',
@@ -415,6 +417,19 @@ export const enTranslation = {
     offlineDirChange: 'Change Directory',
     offlineDirClear: 'Reset to Default',
     offlineDirHint: 'New downloads will use this location. Existing downloads remain at their original path.',
+    hotCacheTitle: 'Hot playback cache',
+    hotCacheAlphaBadge: 'Alpha',
+    hotCacheDisclaimer: 'Preloads upcoming queue tracks and keeps previous ones. When enabled, uses disk space and network.',
+    hotCacheDirDefault: 'Default (App Data)',
+    hotCacheDirChange: 'Change folder',
+    hotCacheDirClear: 'Reset to default',
+    hotCacheDirHint: 'Changing the folder resets the in-app index; files already on disk stay where they are until you remove them.',
+    hotCacheEnabled: 'Enable hot playback cache',
+    hotCacheMaxMb: 'Maximum cache size',
+    hotCacheDebounce: 'Queue change debounce',
+    hotCacheDebounceImmediate: 'Immediate',
+    hotCacheDebounceSeconds: '{{n}} s',
+    hotCacheClearBtn: 'Clear hot cache',
     showArtistImages: 'Show Artist Images',
     showArtistImagesDesc: 'Load and display artist images in the Artists overview. Disabled by default to reduce server disk I/O and network load on large libraries.',
     showTrayIcon: 'Show Tray Icon',