feat(mentions): @ autocomplete, blue @nick, avatar notification toast

- Chat input @ autocomplete: typing @ shows online users (room users +
  online friends + room aliases) with avatars; arrows/Tab/Enter to pick.
- Any valid @nick token is highlighted blue in chat bubbles (like @all),
  giving visual feedback that it is a recognised mention.
- Side notification toast on a received mention: sender avatar (from the
  new senderFigure wire field) + message + dismiss; dismiss marks it read
  so the toolbar unread badge updates. Auto-hides after 8s.
- IMentionEntry/parsers carry senderFigure end to end.

src/hooks/mentions/index.ts

@@ -1,2 +1,3 @@
 export * from './useMentionsSnapshot';
 export * from './useMentionMessages';
+export * from './useMentionAutocomplete';