feat: 浏览器扩展支持交互式页面操作 (browser_act)

在原有只读抓取 (browser_fetch_page) 基础上，新增 observe→act 循环，
让 Agent 能在用户的 Chrome 中实际操作网页：点击/输入/滚动/等待/页内跳转。

后端：
- 新工具 browser_act，按 session 复用标签页，返回带编号的可交互元素清单+正文
- browser_context 任务队列扩展 kind/session_id/action 与 observation 结果
- /api/browser-context 路由下发 act 任务、接收 observation
- BrowserContextConfig 增加 actions_enabled 等开关，tools.py 两级门控
- 接线 app.py 路由挂载、app_config.py 配置字段、config.yaml 工具注册

前端：
- ChatPage 处理「发送当前页」回跳的 browser_context query 参数并自动提交

扩展 (browser-extension/deer-web-clipper)：
- shared.js 自包含注入函数 deerObservePage / deerPerformAction
- background.js 按 kind 分发任务，session→tab 映射存 chrome.storage.session

测试：tests/test_browser_act.py（队列往返 + 工具校验 + 模拟扩展成功路径，8 passed）

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

offline-backend-20260512/backend/app/gateway/app.py

@@ -29,9 +29,10 @@ from app.gateway.routers import (
     ai_writing,
     admin_users,
     agents,
-    artifacts,
-    assistants_compat,
-    auth,
+    artifacts,
+    assistants_compat,
+    auth,
+    browser_context,
     channels,
     checkpoint_migration,
     feedback,
@@ -1197,11 +1198,14 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
     # Artifacts API is mounted at /api/threads/{thread_id}/artifacts
     app.include_router(artifacts.router)
 
-    # Uploads API is mounted at /api/threads/{thread_id}/uploads
-    app.include_router(uploads.router)
-
-    # Thread cleanup API is mounted at /api/threads/{thread_id}
-    app.include_router(threads.router)
+    # Uploads API is mounted at /api/threads/{thread_id}/uploads
+    app.include_router(uploads.router)
+
+    # Optional Chrome-extension page capture API is mounted at /api/browser-context
+    app.include_router(browser_context.router)
+
+    # Thread cleanup API is mounted at /api/threads/{thread_id}
+    app.include_router(threads.router)
 
     # Agents API is mounted at /api/agents
     app.include_router(agents.router)