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/main.tsx

@@ -0,0 +1,49 @@
+import { StrictMode, Component, type ErrorInfo, type ReactNode } from 'react';
+import { createRoot } from 'react-dom/client';
+import 'folds/dist/style.css';
+import { configClass, varsClass } from 'folds';
+import { App } from './App';
+import './styles.css';
+
+document.body.classList.add(configClass, varsClass);
+
+// Playground shares the Cinny origin — drop any SW/caches so stale shims cannot stick.
+if ('serviceWorker' in navigator) {
+  void navigator.serviceWorker.getRegistrations().then((regs) => {
+    for (const reg of regs) void reg.unregister();
+  });
+}
+if (typeof caches !== 'undefined') {
+  void caches.keys().then((keys) => Promise.all(keys.map((k) => caches.delete(k))));
+}
+
+class BootErrorBoundary extends Component<{ children: ReactNode }, { error: string | null }> {
+  state = { error: null as string | null };
+
+  static getDerivedStateFromError(error: Error) {
+    return { error: error.message };
+  }
+
+  componentDidCatch(error: Error, info: ErrorInfo) {
+    console.error('[playground boot]', error, info);
+  }
+
+  render() {
+    if (this.state.error) {
+      return (
+        <pre style={{ padding: 24, color: '#ffb4b4', whiteSpace: 'pre-wrap' }}>
+          {this.state.error}
+        </pre>
+      );
+    }
+    return this.props.children;
+  }
+}
+
+createRoot(document.getElementById('root')!).render(
+  <StrictMode>
+    <BootErrorBoundary>
+      <App />
+    </BootErrorBoundary>
+  </StrictMode>
+);