fix: Random mode now stops after all songs played instead of looping forever

In Random (shuffle) mode, the queue's next() method would return the same
song indefinitely when only one song was in the playlist, and never terminate
even with multiple songs. This happened because played songs were not tracked.

Added a playedIndices Set to track which songs have already been played in
Random mode. Once all songs have been played once, next() returns null to
stop playback — matching the expected behavior where Random plays each song
once in random order, while RandomLoop is the mode for infinite shuffling.

https://claude.ai/code/session_01W3ZncxL5VfdZeB4qqYWDmY

src/audio/queue.test.ts

@@ -150,6 +150,32 @@ describe("PlayQueue", () => {
     expect(next).not.toBeNull();
   });
 
+  it("random mode with single song returns null on next", () => {
+    queue.setMode(PlayMode.Random);
+    queue.add(makeSong("1"));
+    queue.play();
+    expect(queue.next()).toBeNull();
+  });
+
+  it("random mode plays each song exactly once then stops", () => {
+    queue.setMode(PlayMode.Random);
+    queue.add(makeSong("A"));
+    queue.add(makeSong("B"));
+    queue.add(makeSong("C"));
+    queue.play();
+    const played = new Set<string>();
+    played.add(queue.current()!.id);
+    for (let i = 0; i < 3; i++) {
+      const song = queue.next();
+      if (!song) break;
+      played.add(song.id);
+    }
+    // All 3 songs should have been played
+    expect(played).toEqual(new Set(["A", "B", "C"]));
+    // next() after all played should return null
+    expect(queue.next()).toBeNull();
+  });
+
   it("random-loop mode never returns null", () => {
     queue.setMode(PlayMode.RandomLoop);
     queue.add(makeSong("1"));