Add authentication features for admin access

- Introduce a new `auth` module with login, logout, and user verification endpoints for a single admin user.
- Update backend routes to require admin authentication for sensitive operations, enhancing security.
- Modify frontend components to include an authentication provider and gate, ensuring only authorized users can access the application.
- Implement a login page for admin access, improving user experience and security management.
- Update API request handling to redirect unauthorized users to the login page, ensuring proper access control.

backend/routes/auth.py

@@ -0,0 +1,56 @@
+"""Single-admin login endpoints."""
+
+from fastapi import APIRouter, Depends, HTTPException, Response, status
+from pydantic import BaseModel
+from services.auth import (
+    AdminUser,
+    authenticate_admin,
+    clear_auth_cookie,
+    create_admin_token,
+    require_admin,
+    set_auth_cookie,
+)
+
+router = APIRouter(prefix="/api/auth", tags=["auth"])
+
+
+class LoginIn(BaseModel):
+    username: str
+    password: str
+
+
+class AdminUserOut(BaseModel):
+    username: str
+    displayName: str
+    role: str
+
+
+def _to_out(user: AdminUser) -> AdminUserOut:
+    return AdminUserOut(
+        username=user.username,
+        displayName=user.display_name,
+        role=user.role,
+    )
+
+
+@router.post("/login", response_model=AdminUserOut)
+async def login(body: LoginIn, response: Response):
+    user = authenticate_admin(body.username, body.password)
+    if not user:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="用户名或密码错误",
+        )
+    set_auth_cookie(response, create_admin_token())
+    return _to_out(user)
+
+
+@router.post("/logout")
+async def logout(response: Response):
+    clear_auth_cookie(response)
+    return {"ok": True}
+
+
+@router.get("/me", response_model=AdminUserOut)
+async def me(user: AdminUser = Depends(require_admin)):
+    return _to_out(user)