feat: chat — role on hover, role in direct header, @mention dropdown

- MessageBubble: role hidden by default, appears on hover of sender name (group/name CSS)
- Direct chat header: shows other user's role (from otherUserRole field added to backend)
- @mention: typing @ triggers filtered user dropdown, click inserts @Name into message
- Hint text updated: '@ — упомянуть'
- Backend: other_user_role added to rooms query

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api.ts

@@ -1342,6 +1342,7 @@ export interface ChatRoom {
   lastMessageAt: string | null
   lastSender: string | null
   otherUserName: string | null
+  otherUserRole: string | null
   otherUserId: string | null
   otherUserLastRead: string | null
 }