feat(web): add enqueueTrack, enqueueTracks, playRadio to player store

enqueueTrack/enqueueTracks append to the queue without disturbing
playback state (no auto-play on append, no state mutation when queue
was non-empty). playRadio fetches /api/radio?seed_track=<id> and
feeds the response into playQueue. Empty response leaves queue alone.

web/src/lib/player/store.test.ts

@@ -279,3 +279,57 @@ describe('player store — shuffle + repeat + audio reports', () => {
     expect(player.error).toBe('network lost');
   });
 });
+
+import { enqueueTrack, enqueueTracks, playRadio } from './store.svelte';
+import * as client from '$lib/api/client';
+
+describe('player store — enqueue + playRadio', () => {
+  test('enqueueTrack appends to a non-empty queue, _index unchanged', () => {
+    playQueue([track('1'), track('2')], 1);
+    enqueueTrack(track('3'));
+    expect(player.queue.length).toBe(3);
+    expect(player.queue[2].id).toBe('3');
+    expect(player.index).toBe(1);
+  });
+
+  test('enqueueTrack on idle (empty queue) sets index=0 and _state stays idle', () => {
+    playQueue([]);
+    expect(player.state).toBe('idle');
+    enqueueTrack(track('1'));
+    expect(player.queue.length).toBe(1);
+    expect(player.index).toBe(0);
+    expect(player.state).toBe('idle');
+  });
+
+  test('enqueueTracks([]) is a no-op', () => {
+    playQueue([track('1')]);
+    enqueueTracks([]);
+    expect(player.queue.length).toBe(1);
+  });
+
+  test('enqueueTracks concatenates non-empty list', () => {
+    playQueue([track('1')]);
+    enqueueTracks([track('2'), track('3')]);
+    expect(player.queue.map((t) => t.id)).toEqual(['1', '2', '3']);
+  });
+
+  test('playRadio fetches /api/radio and feeds tracks into playQueue', async () => {
+    const apiGet = vi.spyOn(client.api, 'get').mockResolvedValueOnce({
+      tracks: [track('seed'), track('next')]
+    });
+    await playRadio('seed');
+    expect(apiGet).toHaveBeenCalledWith('/api/radio?seed_track=seed');
+    expect(player.queue.length).toBe(2);
+    expect(player.index).toBe(0);
+    expect(player.state).toBe('loading');
+    apiGet.mockRestore();
+  });
+
+  test('playRadio with empty response leaves queue alone', async () => {
+    playQueue([track('a')]);
+    const apiGet = vi.spyOn(client.api, 'get').mockResolvedValueOnce({ tracks: [] });
+    await playRadio('seed');
+    expect(player.queue.map((t) => t.id)).toEqual(['a']);
+    apiGet.mockRestore();
+  });
+});