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/utils/hotCacheGate.ts

@@ -0,0 +1,10 @@
+/** When true, hot-cache prefetch must not start new downloads (playback has priority). */
+let deferHotCachePrefetch = false;
+
+export function setDeferHotCachePrefetch(v: boolean): void {
+  deferHotCachePrefetch = v;
+}
+
+export function getDeferHotCachePrefetch(): boolean {
+  return deferHotCachePrefetch;
+}