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>

frontend/src/pages/MainLayout.tsx

@@ -88,13 +88,21 @@ export default function MainLayout() {
 
   async function openOrCreatePrivate(userId: string) {
     try {
-      const { data } = await api.post(`/api/chats/private/${userId}`);
-      const { data: allChats } = await api.get('/api/chats');
-      setChats(allChats);
-      const chat = allChats.find((c: any) => c.id === data.id);
-      if (chat) openChat(chat);
+      const { data: chatData } = await api.post(`/api/chats/private/${userId}`);
       setSearch('');
-    } catch {}
+      // Load full chat detail directly by ID — don't rely on list lookup
+      const { data: fullChatData } = await api.get(`/api/chats/${chatData.id}`);
+      // Also refresh the sidebar chat list
+      api.get('/api/chats').then(r => setChats(r.data)).catch(() => {});
+      // Build chat object to open
+      const chatToOpen = { ...fullChatData, id: chatData.id };
+      setActiveChat(chatToOpen);
+      setFullChat(chatToOpen);
+      setMobileChatOpen(true);
+      setShowInfo(false);
+    } catch (e) {
+      console.error('openOrCreatePrivate:', e);
+    }
   }
 
   const filtered = chats.filter(c =>