feat(web): mount useEventsDispatcher() in root layout

Single mount point alongside useMediaSession. Once called, the
dispatcher's \$effect watchers run for the lifetime of the SPA.

web/src/routes/+layout.svelte

@@ -14,6 +14,7 @@
     player
   } from '$lib/player/store.svelte';
   import { useMediaSession } from '$lib/player/mediaSession.svelte';
+  import { useEventsDispatcher } from '$lib/player/events.svelte';
 
   let { children } = $props<{ children: import('svelte').Snippet }>();
   let audioEl: HTMLAudioElement | undefined = $state();
@@ -64,6 +65,7 @@
   });
 
   useMediaSession();
+  useEventsDispatcher();
 </script>
 
 <audio