Mirrors the @mention autocomplete exactly -- same trigger-detection
logic (factored into a shared detectTriggerQuery helper, parameterized
on '@' vs '#'), same arrow-key/Enter/Tab/Escape keyboard handling, same
dropdown. Suggests rooms the user belongs to, filtered by name prefix,
showing the room's description as a subtitle when it has one.
MentionAutocomplete.css is renamed to ComposerAutocomplete.css with
generic class names (composer-autocomplete-primary/-secondary instead of
-username/-display-name), now shared by both the mention and room-
reference dropdowns instead of being mention-specific.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
