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/defaultLive.tsx

@@ -0,0 +1,43 @@
+/**
+ * Live preview harness — edit freely. Nothing is saved.
+ *
+ * - Import any Cinny module via `@cinny/...`
+ * - Use mocks / PlaygroundProviders from `@playground/mocks`
+ * - `export default` the component to render in the stage
+ */
+import { Message } from '@cinny/app/features/room/message';
+import { MessageLayout } from '@cinny/app/state/settings';
+import { Box, Text } from 'folds';
+import { mocks } from '@playground/mocks';
+
+export default function Preview() {
+  const room = mocks.room;
+  const mEvent = mocks.event;
+
+  return (
+    <Box direction="Column" gap="200" style={{ width: '100%', maxWidth: 520 }}>
+      <Text size="T200" priority="300">
+        Preview harness — swap imports to any `@cinny/...` export
+      </Text>
+      <Message
+        room={room}
+        mEvent={mEvent}
+        collapse={false}
+        highlight={false}
+        messageLayout={MessageLayout.Modern}
+        messageSpacing="400"
+        canDelete
+        canSendReaction
+        canPinEvent
+        hour24Clock={false}
+        dateFormatString="D MMM YYYY"
+        onUserClick={() => undefined}
+        onUsernameClick={() => undefined}
+        onReplyClick={() => undefined}
+        onReactionToggle={() => undefined}
+      >
+        <Text style={{ margin: 0 }}>{mEvent.getContent().body}</Text>
+      </Message>
+    </Box>
+  );
+}