feat(web): events dispatcher posts to /api/events on player transitions

useEventsDispatcher subscribes to player.current/state via \$effect:
on first transition to playing for a new track, POSTs play_started
and caches the id; on track-id change while a play is open, closes
the prior row as play_skipped; on natural completion (state
playing→paused with position >= duration > 0), POSTs play_ended;
on pagehide, navigator.sendBeacon fires a final play_skipped.
Stable per-tab client_id from sessionStorage.

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

@@ -0,0 +1,19 @@
+import { afterEach, beforeEach, describe, expect, test } from 'vitest';
+import { getOrCreateClientId } from './clientId';
+
+beforeEach(() => sessionStorage.clear());
+afterEach(() => sessionStorage.clear());
+
+describe('getOrCreateClientId', () => {
+  test('generates a UUID on first call and persists it in sessionStorage', () => {
+    const id = getOrCreateClientId();
+    expect(id).toMatch(/^[0-9a-f-]{36}$/);
+    expect(sessionStorage.getItem('minstrel.client_id')).toBe(id);
+  });
+
+  test('returns the same id on subsequent calls', () => {
+    const a = getOrCreateClientId();
+    const b = getOrCreateClientId();
+    expect(a).toBe(b);
+  });
+});