fix(player): minor hot-cache eviction, prefetch budget, and settings live size

Small fix for hot playback cache: eviction keeps current and next only;
prefetch up to five tracks when under cap, always fetch the immediate next;
grace for the previous current until debounce; run evict immediately on
MB or folder changes; re-read cap after download; optional Track.size;
live disk usage on Audio settings.

src/utils/hotCacheGate.ts

@@ -1,6 +1,11 @@
 /** When true, hot-cache prefetch must not start new downloads (playback has priority). */
 let deferHotCachePrefetch = false;
 
+/** Last track that was «current» before a forward queue step — not evicted until debounce elapses. */
+let previousTrackGraceUntilMs = 0;
+let previousTrackGraceTrackId: string | null = null;
+let previousTrackGraceServerId: string | null = null;
+
 export function setDeferHotCachePrefetch(v: boolean): void {
   deferHotCachePrefetch = v;
 }
@@ -8,3 +13,26 @@ export function setDeferHotCachePrefetch(v: boolean): void {
 export function getDeferHotCachePrefetch(): boolean {
   return deferHotCachePrefetch;
 }
+
+/** Call when `queueIndex` advances; the old current track stays eviction-safe for `debounceSec` (capped at 600 s). */
+export function bumpHotCachePreviousTrackGrace(
+  trackId: string,
+  serverId: string,
+  debounceSec: number,
+): void {
+  const sec = Number.isFinite(debounceSec) ? Math.min(600, Math.max(0, debounceSec)) : 0;
+  previousTrackGraceUntilMs = Date.now() + sec * 1000;
+  previousTrackGraceTrackId = trackId;
+  previousTrackGraceServerId = serverId;
+}
+
+export function isHotCachePreviousTrackUnderGrace(trackId: string, serverId: string): boolean {
+  if (!previousTrackGraceTrackId || Date.now() >= previousTrackGraceUntilMs) return false;
+  return previousTrackGraceTrackId === trackId && previousTrackGraceServerId === serverId;
+}
+
+export function clearHotCachePreviousGrace(): void {
+  previousTrackGraceUntilMs = 0;
+  previousTrackGraceTrackId = null;
+  previousTrackGraceServerId = null;
+}