refactor(webui): infer fallback from active model

webui/src/tests/nanobot-client.test.ts

@@ -1724,7 +1724,6 @@ describe("NanobotClient", () => {
       chat_id: "chat-a",
       model_name: "deepseek/deepseek-chat",
       model_preset: "Deep Research",
-      is_fallback: true,
     });
 
     expect(chatHandler).toHaveBeenCalledWith({
@@ -1732,7 +1731,6 @@ describe("NanobotClient", () => {
       chat_id: "chat-a",
       model_name: "deepseek/deepseek-chat",
       model_preset: "Deep Research",
-      is_fallback: true,
     });
   });
 

webui/src/tests/thread-shell.test.tsx

@@ -720,7 +720,6 @@ describe("ThreadShell", () => {
         event: "turn_model_updated",
         chat_id: "fallback-model",
         model_name: "openai-codex/gpt-5.5",
-        is_fallback: false,
       });
     });
 
@@ -731,7 +730,6 @@ describe("ThreadShell", () => {
         event: "turn_model_updated",
         chat_id: "fallback-model",
         model_name: "deepseek/deepseek-chat",
-        is_fallback: true,
       });
     });