fix(a11y): stable reaction labels — emoji glyph, custom emoji shortcode (#179)

The reaction button's aria-label used getShortcodeFor(), which returns
undefined until the lazily loaded emoji data arrives. The same button read
"🎉 reaction, 1 person" on first render and "tada reaction, 2 people" after
any later re-render. It now always uses the emoji itself (screen readers
speak it by its proper name, e.g. "party popper").

Custom (mxc) emoji were labelled just "custom emoji"; they now use the
shortcode carried on the reaction event (":lotus_blob: reaction").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/reaction-viewer/ReactionViewer.tsx

@@ -107,6 +107,7 @@ export const ReactionViewer = as<'div', ReactionViewerProps>(
                     key={key}
                     mx={mx}
                     reaction={key}
+                    shortcode={Array.from(evts).find(eventWithShortcode)?.getContent().shortcode}
                     count={evts.size}
                     role="option"
                     aria-selected={key === selectedKey}