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/plugins/react-custom-html-parser.tsx

@@ -188,7 +188,7 @@ export const scaleSystemEmoji = (text: string): (string | JSX.Element)[] =>
     text,
     EMOJI_REG_G,
     (match, pushIndex) => (
-      <span key={`scaleSystemEmoji-${pushIndex}`} className={css.EmoticonBase}>
+      <span key={`scaleSystemEmoji-${pushIndex}`} className={css.EmoticonBase} data-emoticon={match[0]}>
         <span className={css.Emoticon()} title={getShortcodeFor(getHexcodeForEmoji(match[0]))}>
           {match[0]}
         </span>
@@ -551,8 +551,15 @@ export const getReactCustomHtmlParser = (
             );
           }
           if (htmlSrc && 'data-mx-emoticon' in props) {
+            const emoticonLabel =
+              (typeof props.alt === 'string' && props.alt) ||
+              (typeof props.title === 'string' && props.title) ||
+              undefined;
             return (
-              <span className={css.EmoticonBase}>
+              <span
+                className={css.EmoticonBase}
+                {...(emoticonLabel ? { 'data-emoticon': emoticonLabel } : {})}
+              >
                 <span className={css.Emoticon()}>
                   <AuthenticatedImg {...props} className={css.EmoticonImg} src={htmlSrc} />
                 </span>