Add emoji shortcode autocomplete to the composer (#54)

Typing ":name" now shows a matching-shortcode dropdown (same
join/leave/arrow-key UX as the existing @mention and #room autocompletes),
selecting one inserts the actual glyph immediately rather than leaving
literal ":name:" text. A bare ":" with nothing typed yet suggests
recently-used emoji instead of an arbitrary slice of the ~950 known
shortcodes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/ComposerAutocomplete.css

@@ -40,6 +40,11 @@
   color: var(--ds-accent);
 }
 
+.composer-autocomplete-emoji-glyph {
+  font-size: 1rem;
+  line-height: 1;
+}
+
 .composer-autocomplete-secondary {
   font-size: 0.76rem;
   color: var(--ds-muted);