fix: open private chat from sidebar search

- openOrCreatePrivate: load chat by ID directly (skip stale list lookup)
- GET /api/chats/🆔 resolve title/avatar/privateUserId for private chats
  (was returning null title, causing chat area to behave unexpectedly)

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

backend/src/routes/chats.ts

@@ -89,17 +89,21 @@ export default async function chatRoutes(app: FastifyInstance) {
 
     const onlineSet = new Set(connections.keys());
 
+    // For private chats, resolve title/avatar/color from the other member
+    const other = chat.type === 'private' ? members.find((m: any) => m.id !== userId) : null;
+
     return {
       id: chat.id,
       type: chat.type,
-      title: chat.title,
+      title: chat.type === 'private' ? (other?.display_name || '') : chat.title,
       description: chat.description,
-      avatarColor: chat.avatar_color,
-      avatar: chat.avatar || null,
+      avatarColor: chat.type === 'private' ? (other?.avatar_color || chat.avatar_color) : chat.avatar_color,
+      avatar: chat.type === 'private' ? (other?.avatar || null) : (chat.avatar || null),
       isPublic: chat.is_public,
       myRole: member.role,
       canSendMessages: member.can_send_messages,
       canAddMembers: member.can_add_members,
+      privateUserId: chat.type === 'private' ? (other?.id || null) : null,
       createdAt: chat.created_at,
       members: members.map(m => ({
         id: m.id, username: m.username, displayName: m.display_name,