feat: 支持多语言提示词本地化和界面优化

- 添加 prompt_locale 参数支持简体中文、繁体中文和英文提示词本地化
- 移除内置 agents 配置以简化系统架构
- 更新 ContextBuilder 使用动态提示词模板而非硬编码内容
- 在 AgentLoop、Web 接口和 AgentService 中传递 locale 参数
- 添加输出语言指令确保用户界面内容按指定语言生成
- 扩展前端 LanguageSwitcher 组件支持三种语言选项
- 优化 Header 和侧边栏组件的响应式布局和文本截断处理
- 更新测试用例验证不同语言环境下的提示词正确性

app-instance/frontend/lib/task-timeline.test.ts

@@ -143,6 +143,48 @@ describe('buildTaskTimelineCards', () => {
     expect(cards[6].relatedArtifactIds).toEqual(['artifact-summary']);
   });
 
+  it('localizes generated milestone titles for English and Traditional Chinese', () => {
+    const task = makeTask();
+    const processEvents: ProcessEvent[] = [
+      {
+        event_id: 'evt-plan',
+        run_id: 'run-main',
+        parent_run_id: null,
+        kind: 'task_planned',
+        actor_type: 'agent',
+        actor_id: 'planner',
+        actor_name: 'Task Planner',
+        text: 'Plan created.',
+        created_at: '2026-05-26T10:01:00.000Z',
+      },
+      {
+        event_id: 'evt-tool-start',
+        run_id: 'run-main',
+        parent_run_id: null,
+        kind: 'tool_call_started',
+        actor_type: 'mcp',
+        actor_id: 'user_files_list',
+        actor_name: 'user_files_list',
+        text: 'Calling tool: user_files_list.',
+        created_at: '2026-05-26T10:02:00.000Z',
+      },
+    ];
+
+    const englishCards = buildTaskTimelineCards({ task, processEvents, locale: 'en-US' });
+    const traditionalCards = buildTaskTimelineCards({ task, processEvents, locale: 'zh-Hant' });
+
+    expect(englishCards.map((card) => card.title)).toEqual([
+      'Task created',
+      'Execution plan',
+      'Calling tool: user_files_list',
+    ]);
+    expect(traditionalCards.map((card) => card.title)).toEqual([
+      '任務已創建',
+      '執行計劃',
+      '調用工具：user_files_list',
+    ]);
+  });
+
   it('appends result and acceptance cards for closed tasks with feedback', () => {
     const task = makeTask({
       is_open: false,