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/LivePreview.tsx

@@ -0,0 +1,59 @@
+import { useEffect, useState } from 'react';
+
+/**
+ * Dynamically remounts the Vite virtual live module when the harness changes.
+ */
+export function LivePreview() {
+  const [revision, setRevision] = useState(0);
+  const [Comp, setComp] = useState<React.ComponentType | null>(null);
+  const [error, setError] = useState<string | null>(null);
+
+  useEffect(() => {
+    const onUpdate = () => setRevision((n) => n + 1);
+    const handler = () => onUpdate();
+
+    // Vite custom event from liveTsxPlugin
+    if (import.meta.hot) {
+      import.meta.hot.on('playground:live-updated', handler);
+    }
+
+    return () => {
+      if (import.meta.hot) {
+        import.meta.hot.off('playground:live-updated', handler);
+      }
+    };
+  }, []);
+
+  useEffect(() => {
+    let cancelled = false;
+    setError(null);
+    setComp(null);
+
+    import(/* @vite-ignore */ `/@playground/live.tsx?t=${revision}`)
+      .then((mod) => {
+        if (cancelled) return;
+        const candidate = mod.default;
+        if (typeof candidate !== 'function') {
+          setError('Live module must `export default` a React component.');
+          return;
+        }
+        setComp(() => candidate);
+      })
+      .catch((err: unknown) => {
+        if (cancelled) return;
+        const message =
+          err instanceof Error
+            ? `${err.message}\n\n${err.stack ?? ''}`
+            : String(err);
+        setError(message);
+      });
+
+    return () => {
+      cancelled = true;
+    };
+  }, [revision]);
+
+  if (error) return <pre className="error-block">{error}</pre>;
+  if (!Comp) return <div className="muted">Compiling harness…</div>;
+  return <Comp />;
+}