feat(player): playNext for M7 #372 track-actions menu

Inserts at _queue[_index + 1] so the next-up slot is overwritten with
the chosen track. Empty-queue seeding mirrors enqueueTrack's behavior.
Clears _radioSeedId since user-driven enqueue invalidates the M4c
auto-refresh trigger.

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

@@ -211,6 +211,22 @@ export function enqueueTrack(t: TrackRef): void {
   if (_state === 'idle') _index = 0;
 }
 
+/**
+ * M7 #372: insert a track immediately after the currently-playing one
+ * so it plays next. If the queue is empty, behaves like enqueueTrack
+ * and starts the queue with this track at index 0.
+ */
+export function playNext(t: TrackRef): void {
+  _radioSeedId = null; // M4c: any user-driven enqueue clears the radio refresh state
+  if (_queue.length === 0) {
+    _queue = [t];
+    _index = 0;
+    return;
+  }
+  const next = _index + 1;
+  _queue = [..._queue.slice(0, next), t, ..._queue.slice(next)];
+}
+
 export function enqueueTracks(ts: TrackRef[]): void {
   _radioSeedId = null;  // M4c
   if (ts.length === 0) return;

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

@@ -280,7 +280,7 @@ describe('player store — shuffle + repeat + audio reports', () => {
   });
 });
 
-import { enqueueTrack, enqueueTracks, playRadio } from './store.svelte';
+import { enqueueTrack, enqueueTracks, playNext, playRadio } from './store.svelte';
 import * as client from '$lib/api/client';
 
 describe('player store — enqueue + playRadio', () => {
@@ -334,6 +334,22 @@ describe('player store — enqueue + playRadio', () => {
   });
 });
 
+describe('playNext', () => {
+  test('splices at _index + 1, leaving _index unchanged', () => {
+    playQueue([track('a'), track('b')], 0);
+    playNext(track('c'));
+    expect(player.queue.map((t) => t.id)).toEqual(['a', 'c', 'b']);
+    expect(player.index).toBe(0);
+  });
+
+  test('on empty queue, seeds with the track at index 0', () => {
+    playQueue([]);
+    playNext(track('x'));
+    expect(player.queue.map((t) => t.id)).toEqual(['x']);
+    expect(player.index).toBe(0);
+  });
+});
+
 describe('M4c radio auto-refresh at 80%', () => {
   test('refresh fires at 80% queue consumption', async () => {
     const tracks = [track('seed'), track('t1'), track('t2'), track('t3'), track('t4')];