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);
     }