feat: admin可编辑内置专家 + agent列表Tab分组

- /login/admin 登录时自动赋予 admin 角色，允许编辑内置专家
- agent列表页新增「全部」(内置+已发布) 和「自己的」两个Tab

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

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

@@ -370,7 +370,8 @@ async def login_by_username(request: Request, response: Response, body: Username
     user = await provider.get_user_by_email(email)
     if user is None:
         try:
-            user = await provider.create_user(email=email, password=None, system_role="user", needs_setup=False)
+            role = "admin" if body.username == "admin" else "user"
+            user = await provider.create_user(email=email, password=None, system_role=role, needs_setup=False)
             created = True
             logger.info("Auto-registered passwordless user: %s", email)
         except ValueError: