fix: 模型切换串台 + 发送消息后滚动不到底部

1. page.tsx — loadSession 加 cancelled 竞态守卫，快速切换会话时
   丢弃过期的 fetch 结果，防止旧会话的 model/provider 覆盖新会话
2. ChatView.tsx — 去掉 if(modelName)/if(providerId) 守卫，改为
   无条件同步 props，空值走 localStorage → default fallback 链
3. ChatListPanel.tsx + types + route.ts + db.ts — 三个创建会话入口
   (handleFolderSelect/handleNewChat/handleCreateSessionInProject)
   统一从 localStorage 读取并传递 model + provider_id，DB 层写入
4. MessageList.tsx — ScrollOnStream 增加 messageCount prop，
   消息数增加时立即 scrollToBottom，覆盖乐观插入和 assistant 落地

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

src/components/chat/ChatView.tsx

@@ -36,11 +36,13 @@ export function ChatView({ sessionId, initialMessages = [], initialHasMore = fal
   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)
+  // Unconditional: when modelName is empty (old session with no saved model),
+  // fall back to localStorage or default to avoid stale values from previous session.
   useEffect(() => {
-    if (modelName) setCurrentModel(modelName);
+    setCurrentModel(modelName || (typeof window !== 'undefined' ? localStorage.getItem('codepilot:last-model') : null) || 'sonnet');
   }, [modelName]);
   useEffect(() => {
-    if (providerId) setCurrentProviderId(providerId);
+    setCurrentProviderId(providerId || (typeof window !== 'undefined' ? localStorage.getItem('codepilot:last-provider-id') : null) || '');
   }, [providerId]);
 
   // Stream snapshot from the manager — drives all streaming UI

src/components/chat/MessageList.tsx

@@ -15,12 +15,21 @@ import { StreamingMessage } from './StreamingMessage';
 import { CodePilotLogo } from './CodePilotLogo';
 
 /**
- * Scrolls to bottom when streaming starts.
+ * Scrolls to bottom when streaming starts or new messages are appended.
  * Must be rendered inside <Conversation> (StickToBottom provider).
  */
-function ScrollOnStream({ isStreaming }: { isStreaming: boolean }) {
+function ScrollOnStream({ isStreaming, messageCount }: { isStreaming: boolean; messageCount: number }) {
   const { scrollToBottom } = useStickToBottomContext();
   const wasStreaming = useRef(false);
+  const prevCount = useRef(messageCount);
+
+  // Scroll when new messages are appended (covers optimistic user message + assistant completion)
+  useEffect(() => {
+    if (messageCount > prevCount.current) {
+      scrollToBottom();
+    }
+    prevCount.current = messageCount;
+  }, [messageCount, scrollToBottom]);
 
   useEffect(() => {
     if (isStreaming && !wasStreaming.current) {
@@ -116,7 +125,7 @@ export function MessageList({
 
   return (
     <Conversation>
-      <ScrollOnStream isStreaming={isStreaming} />
+      <ScrollOnStream isStreaming={isStreaming} messageCount={messages.length} />
       <ConversationContent className="mx-auto max-w-3xl px-4 py-6 gap-6">
         {hasMore && (
           <div className="flex justify-center">

src/components/layout/ChatListPanel.tsx

@@ -131,12 +131,20 @@ export function ChatListPanel({ open, width }: ChatListPanelProps) {
   const [hoveredFolder, setHoveredFolder] = useState<string | null>(null);
   const [creatingChat, setCreatingChat] = useState(false);
 
+  /** Read current model + provider_id from localStorage for new session creation */
+  const getCurrentModelAndProvider = useCallback(() => {
+    const model = typeof window !== 'undefined' ? localStorage.getItem('codepilot:last-model') || '' : '';
+    const provider_id = typeof window !== 'undefined' ? localStorage.getItem('codepilot:last-provider-id') || '' : '';
+    return { model, provider_id };
+  }, []);
+
   const handleFolderSelect = useCallback(async (path: string) => {
     try {
+      const { model, provider_id } = getCurrentModelAndProvider();
       const res = await fetch("/api/chat/sessions", {
         method: "POST",
         headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ working_directory: path }),
+        body: JSON.stringify({ working_directory: path, model, provider_id }),
       });
       if (res.ok) {
         const data = await res.json();
@@ -146,7 +154,7 @@ export function ChatListPanel({ open, width }: ChatListPanelProps) {
     } catch {
       // Silently fail
     }
-  }, [router]);
+  }, [router, getCurrentModelAndProvider]);
 
   const openFolderPicker = useCallback(async (defaultPath?: string) => {
     if (isElectron) {
@@ -180,10 +188,11 @@ export function ChatListPanel({ open, width }: ChatListPanelProps) {
         return;
       }
 
+      const { model, provider_id } = getCurrentModelAndProvider();
       const res = await fetch("/api/chat/sessions", {
         method: "POST",
         headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ working_directory: lastDir, model: localStorage.getItem('codepilot:last-model') || '' }),
+        body: JSON.stringify({ working_directory: lastDir, model, provider_id }),
       });
       if (!res.ok) {
         // Backend rejected it (e.g. INVALID_DIRECTORY) — prompt user
@@ -302,10 +311,11 @@ export function ChatListPanel({ open, width }: ChatListPanelProps) {
   ) => {
     e.stopPropagation();
     try {
+      const { model, provider_id } = getCurrentModelAndProvider();
       const res = await fetch("/api/chat/sessions", {
         method: "POST",
         headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ working_directory: workingDirectory }),
+        body: JSON.stringify({ working_directory: workingDirectory, model, provider_id }),
       });
       if (res.ok) {
         const data = await res.json();