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/components/message/layout/layout.css.ts

@@ -243,6 +243,9 @@ export const MessageTextBody = recipe({
         overflow: 'visible',
         overflowY: 'visible',
         paddingBottom: config.space.S200,
+        cursor: 'pointer',
+        position: 'relative',
+        zIndex: 5,
       },
     },
     emote: {
@@ -263,6 +266,8 @@ globalStyle(`${jumboEmojiClass} .${htmlCss.EmoticonBase}`, {
   padding: 0,
   overflow: 'visible',
   verticalAlign: 'middle',
+  position: 'relative',
+  zIndex: 1,
 });
 
 globalStyle(`${jumboEmojiClass} .${htmlCss.Emoticon.classNames.base}`, {