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>

backend/src/routes/chat.ts

@@ -95,6 +95,7 @@ const chatRoutes: FastifyPluginAsync = async (fastify) => {
                 (SELECT m.created_at FROM chat_messages m WHERE m.room_id = r.id ORDER BY m.created_at DESC LIMIT 1) AS last_message_at,
                 (SELECT COALESCE(m.system_name, u.name) FROM chat_messages m LEFT JOIN users u ON u.id = m.sender_id WHERE m.room_id = r.id ORDER BY m.created_at DESC LIMIT 1) AS last_sender,
                 (SELECT u.name FROM chat_room_members crm JOIN users u ON u.id = crm.user_id WHERE crm.room_id = r.id AND crm.user_id != $2 LIMIT 1) AS other_user_name,
+                (SELECT u.role FROM chat_room_members crm JOIN users u ON u.id = crm.user_id WHERE crm.room_id = r.id AND crm.user_id != $2 LIMIT 1) AS other_user_role,
                 (SELECT crm.user_id FROM chat_room_members crm WHERE crm.room_id = r.id AND crm.user_id != $2 LIMIT 1) AS other_user_id,
                 (SELECT rs.last_read FROM chat_read_status rs WHERE rs.room_id = r.id AND rs.user_id != $2 LIMIT 1) AS other_user_last_read
            FROM chat_rooms r