fix: persist model on conversation send, fallback resolution, carry model on new conversation

ui/src/api.ts

@@ -194,8 +194,8 @@ export const api = {
   settingsSet: (key: string, value: string) =>
     invoke<void>("settings_set", { key, value }),
   listConversations: () => invoke<Conversation[]>("list_conversations"),
-  createConversation: (title: string) =>
-    invoke<Conversation>("create_conversation", { title }),
+  createConversation: (title: string, modelId?: string) =>
+    invoke<Conversation>("create_conversation", { title, modelId: modelId ?? null }),
   deleteConversation: (id: string) =>
     invoke<void>("delete_conversation", { id }),
   getMessages: (conversationId: string) =>

ui/src/pages/ChatPage.tsx

@@ -331,7 +331,7 @@ export default function ChatPage() {
   }
 
   async function handleNewConversation() {
-    const conv = await api.createConversation("新会话");
+    const conv = await api.createConversation("新会话", selectedModelId || undefined);
     setActiveConvId(conv.id);
     setMessages([]);
     setError(null);