fix: prevent horizontal overflow on mobile in /play layout

Add overflow-hidden to outer wrapper so the absolutely-positioned
SexyBackground is clipped, matching how public pages handle it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/play/+layout.svelte

@@ -41,7 +41,9 @@
   const displayName = $derived(user.artist_name ?? user.email);
 </script>
 
-<div class="min-h-screen bg-gradient-to-br from-background via-primary/5 to-accent/5 relative">
+<div
+  class="min-h-screen bg-gradient-to-br from-background via-primary/5 to-accent/5 relative overflow-hidden"
+>
   <SexyBackground />
 
   <div class="container mx-auto px-4 relative z-10">