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.ts

@@ -0,0 +1,13 @@
+const KEY = 'minstrel.client_id';
+
+export function getOrCreateClientId(): string {
+  try {
+    const existing = sessionStorage.getItem(KEY);
+    if (existing) return existing;
+  } catch {
+    // sessionStorage unavailable (non-browser env). Generate ephemerally.
+  }
+  const id = crypto.randomUUID();
+  try { sessionStorage.setItem(KEY, id); } catch { /* ignore */ }
+  return id;
+}