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/components/room/widgets/chat/highlightMentions.tsx

@@ -51,9 +51,14 @@ const isMentionToken = (token: string, ownUsernameLower: string, aliases: Readon
 
     if(!nick) return false;
 
+    // Own nick and room-broadcast aliases always count.
     if(ownUsernameLower && nick === ownUsernameLower) return true;
+    if(aliases.has(nick)) return true;
 
-    return aliases.has(nick);
+    // Any other valid @nick token is also highlighted (blue), so a direct
+    // @username mention reads the same as @all — visual feedback that it is a
+    // recognised mention. (Cosmetic only; the server decides actual delivery.)
+    return true;
 };
 
 /**