Add emoji confetti bursts and component playground.

Support app.relay.emoji_confetti on jumbo emoji clicks with canvas physics,
per-emoji particle profiles, and a fixed overlay that does not affect scroll.
Also add a Vite playground for live component previews and theme hover preview.

src/playground/defaultSource.ts

@@ -0,0 +1,3 @@
+import defaultLive from './defaultLive.tsx?raw';
+
+export const DEFAULT_SOURCE = defaultLive;