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/KiraAvatar.tsx

@@ -197,7 +197,7 @@ export default function KiraAvatar(props: Props) {
   `;
 
   return (
-    <div className="glass-card flex flex-col items-center w-full overflow-hidden" style={{ minHeight: '33vh' }}>
+    <div className="flex flex-col items-center w-full overflow-hidden" style={{ minHeight: '33vh' }}>
       <div className="relative w-full flex-1" style={{ minHeight: 250 }}>
         <div
           ref={canvasRef}