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.svelte.ts

@@ -1,4 +1,5 @@
-import type { TrackRef } from '$lib/api/types';
+import type { TrackRef, RadioResponse } from '$lib/api/types';
+import { api } from '$lib/api/client';
 
 export type PlayerState = 'idle' | 'loading' | 'playing' | 'paused' | 'error';
 export type RepeatMode = 'off' | 'all' | 'one';
@@ -196,3 +197,21 @@ export function reportStateFromAudio(
       return;
   }
 }
+
+export function enqueueTrack(t: TrackRef): void {
+  _queue = [..._queue, t];
+  if (_state === 'idle') _index = 0;
+}
+
+export function enqueueTracks(ts: TrackRef[]): void {
+  if (ts.length === 0) return;
+  _queue = [..._queue, ...ts];
+  if (_state === 'idle') _index = 0;
+}
+
+export async function playRadio(seedTrackId: string): Promise<void> {
+  const resp = await api.get<RadioResponse>(
+    `/api/radio?seed_track=${encodeURIComponent(seedTrackId)}`
+  );
+  if (resp.tracks.length > 0) playQueue(resp.tracks, 0);
+}