feat(web): subscribe to SSE and refresh home/playlists on playlist.system_rebuilt

The web client only ever SENT events; it had no inbound SSE listener, so a
tab left open across the daily system-playlist rebuild kept showing
yesterday's home + playlist snapshots until a manual reload (the stale-
browse-view bug behind #968). Add useServerEvents(): opens /api/events/stream
while authenticated and, on playlist.system_rebuilt, invalidates the home,
playlists, and system-playlist-status query caches. Deliberately does not
disturb the active playback queue — that self-heals on the failure path.
Issue #968.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/routes/+layout.svelte

@@ -23,6 +23,7 @@
   } from '$lib/player/store.svelte';
   import { useMediaSession } from '$lib/player/mediaSession.svelte';
   import { useEventsDispatcher } from '$lib/player/events.svelte';
+  import { useServerEvents } from '$lib/serverEvents.svelte';
   import { useGlobalShortcuts } from '$lib/player/shortcuts.svelte';
   import { applyMetaThemeColor } from '$lib/theme/applyMetaThemeColor.svelte';
   import { audioLoader } from '$lib/player/audioLoader';
@@ -132,6 +133,7 @@
 
   useMediaSession();
   useEventsDispatcher();
+  useServerEvents();
   useGlobalShortcuts();
   applyMetaThemeColor();
 </script>