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/app/features/room/emoji-confetti/findJumboMount.ts

@@ -0,0 +1,11 @@
+export function findJumboEmojiElement(targetEventId: string): HTMLElement | null {
+  const message = document.querySelector(`[data-message-id="${CSS.escape(targetEventId)}"]`);
+  if (!message) return null;
+
+  const jumbo = message.querySelector('[data-jumbo-emoji]');
+  return jumbo instanceof HTMLElement ? jumbo : null;
+}
+
+export function getLocalBurstCanvasSize(maskRadius: number): number {
+  return Math.max(300, Math.round(maskRadius * 6.5));
+}