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/PreviewErrorBoundary.tsx

@@ -0,0 +1,40 @@
+import React, { Component, type ErrorInfo, type ReactNode } from 'react';
+
+type Props = {
+  resetKey: string;
+  children: ReactNode;
+};
+
+type State = {
+  error: string | null;
+};
+
+export class PreviewErrorBoundary extends Component<Props, State> {
+  state: State = { error: null };
+
+  static getDerivedStateFromError(error: Error): State {
+    return {
+      error: `${error.message}\n\n${error.stack ?? ''}`,
+    };
+  }
+
+  componentDidCatch(error: Error, info: ErrorInfo) {
+    console.error('[playground preview]', error, info);
+    this.setState({
+      error: `${error.message}\n\n${error.stack ?? ''}\n\n${info.componentStack ?? ''}`,
+    });
+  }
+
+  componentDidUpdate(prevProps: Props) {
+    if (prevProps.resetKey !== this.props.resetKey && this.state.error) {
+      this.setState({ error: null });
+    }
+  }
+
+  render() {
+    if (this.state.error) {
+      return <pre className="error-block">{this.state.error}</pre>;
+    }
+    return this.props.children;
+  }
+}