feat(ui): ditch all glass-card containers — flat, modern, card-free layout

All 15+ glass-card instances removed across every component (Timer, Music,
Notes, WhiteNoise, PetZone, Clock, ChatBubble, Wardrobe, Toolbar, KiraAvatar,
BackgroundScene, WelcomeScreen, App text input + bottom bar).

New design: widgets sit directly on the gradient background with only padding,
no frosted-glass backgrounds, borders, or shadows. Cleaner, more modern look.

frontend/src/components/ChatBubble.tsx

@@ -22,7 +22,7 @@ export default function ChatBubble({ messages, isKiraSpeaking, livePartial }: Pr
   }, [messages]);
 
   return (
-    <div className="glass-card p-4 flex flex-col" style={{ minHeight: 200, maxHeight: 320 }}>
+    <div className="p-4 flex flex-col" style={{ minHeight: 200, maxHeight: 320 }}>
       <h3 className="text-sm font-bold text-kira-plum mb-3 flex items-center gap-2">
         <span>💬</span> Conversation
         <span className={`w-2 h-2 rounded-full ${isKiraSpeaking ? 'bg-kira-pink animate-pulse' : 'bg-kira-mint'}`} />