fix(queue): restore playedIndices clear in playAt; test HISTORY_LIMIT

Code review of 390d3fa flagged that dropping playedIndices.clear() from
playAt is an unnecessary behavior change. The two operations (push
history, reset random pool) are not in tension — restoring the clear
preserves the original 'explicit pick restarts shuffle' semantic that
Random mode users rely on, while still tracking the prev-history stack.

Also add a regression test that the 50-entry HISTORY_LIMIT cap works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/audio/queue.ts

@@ -86,6 +86,10 @@ export class PlayQueue {
   playAt(index: number): QueuedSong | null {
     if (index < 0 || index >= this.songs.length) return null;
     this.pushHistory(this.currentIndex);
+    // Reset the Random-mode "unplayed" pool — explicit picks restart
+    // shuffle from this point. History tracking is independent and
+    // unaffected by this clear.
+    this.playedIndices.clear();
     this.currentIndex = index;
     this.playedIndices.add(index);
     return this.songs[index];