fix: 切换聊天后模型选择不保持

两个问题：
1. 用户选择模型只写 localStorage，不写 DB。切换聊天回来后从 DB
   读到空值，模型回退到默认值
2. ChatView 的 useState 只用初始值，后续 modelName prop 异步更新
   不会同步到 currentModel state

修复：
- handleProviderModelChange 选择模型时立即 PATCH 到 DB
- PATCH /api/chat/sessions/:id 新增 model + provider_id 字段支持
- ChatView 添加 useEffect 同步 modelName/providerId prop 变化

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

src/app/api/chat/sessions/[id]/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest } from 'next/server';
-import { deleteSession, getSession, updateSessionWorkingDirectory, updateSessionTitle, updateSessionMode, clearSessionMessages } from '@/lib/db';
+import { deleteSession, getSession, updateSessionWorkingDirectory, updateSessionTitle, updateSessionMode, updateSessionModel, updateSessionProviderId, clearSessionMessages } from '@/lib/db';
 
 export async function GET(
   _request: NextRequest,
@@ -40,6 +40,12 @@ export async function PATCH(
     if (body.mode) {
       updateSessionMode(id, body.mode);
     }
+    if (body.model !== undefined) {
+      updateSessionModel(id, body.model);
+    }
+    if (body.provider_id !== undefined) {
+      updateSessionProviderId(id, body.provider_id);
+    }
     if (body.clear_messages) {
       clearSessionMessages(id);
     }

src/components/chat/ChatView.tsx

@@ -35,6 +35,14 @@ export function ChatView({ sessionId, initialMessages = [], initialHasMore = fal
   const [currentModel, setCurrentModel] = useState(modelName || (typeof window !== 'undefined' ? localStorage.getItem('codepilot:last-model') : null) || 'sonnet');
   const [currentProviderId, setCurrentProviderId] = useState(providerId || (typeof window !== 'undefined' ? localStorage.getItem('codepilot:last-provider-id') : null) || '');
 
+  // Sync model/provider when session data loads (props update after async fetch)
+  useEffect(() => {
+    if (modelName) setCurrentModel(modelName);
+  }, [modelName]);
+  useEffect(() => {
+    if (providerId) setCurrentProviderId(providerId);
+  }, [providerId]);
+
   // Stream snapshot from the manager — drives all streaming UI
   const [streamSnapshot, setStreamSnapshot] = useState<SessionStreamSnapshot | null>(
     () => getSnapshot(sessionId)
@@ -79,7 +87,13 @@ export function ChatView({ sessionId, initialMessages = [], initialHasMore = fal
   const handleProviderModelChange = useCallback((newProviderId: string, model: string) => {
     setCurrentProviderId(newProviderId);
     setCurrentModel(model);
-  }, []);
+    // Persist immediately so switching chats preserves the selection
+    fetch(`/api/chat/sessions/${sessionId}`, {
+      method: 'PATCH',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ model, provider_id: newProviderId }),
+    }).catch(() => {});
+  }, [sessionId]);
 
   // Subscribe to stream-session-manager for this session.
   // On unmount we only unsubscribe — we do NOT abort the stream.