fix: use Random mode for FM instead of RandomLoop to prevent song repeats

RandomLoop randomly selects from queue, causing the same song to play
multiple times when the queue is small (~3 songs from personal_fm API).
Random mode ensures each song plays once before repeating, relying on
refillFm() to fetch fresh songs before exhaustion.

Also fix the proactive refill condition to use unplayedCount() instead
of size() - getCurrentIndex(), which was meaningless for non-sequential
play modes.

src/audio/queue.ts

@@ -161,4 +161,9 @@ export class PlayQueue {
   getCurrentIndex(): number {
     return this.currentIndex;
   }
+
+  /** Number of songs not yet played in Random mode. */
+  unplayedCount(): number {
+    return this.songs.length - this.playedIndices.size;
+  }
 }