feat(web): pre-fill "system" user ID on login form with hint note

Default User ID to "system" and add i18n hint text explaining it
grants full system access, reducing friction for admin login.

ui/web/src/i18n/locales/en/login.json

@@ -13,6 +13,7 @@
   "token": {
     "userId": "User ID",
     "userIdPlaceholder": "your-user-id",
+    "userIdHint": "Use \"system\" for full system access",
     "gatewayToken": "Gateway Token",
     "tokenPlaceholder": "Bearer token",
     "connect": "Connect",

ui/web/src/i18n/locales/vi/login.json

@@ -33,7 +33,8 @@
     "gatewayToken": "Gateway Token",
     "tokenPlaceholder": "Bearer token",
     "userId": "User ID",
-    "userIdPlaceholder": "your-user-id"
+    "userIdPlaceholder": "your-user-id",
+    "userIdHint": "Dùng \"system\" để có toàn quyền hệ thống"
   },
   "noAccessHint": "Yêu cầu quản trị viên thêm bạn vào tenant, hoặc đăng nhập bằng tài khoản khác.",
   "logout": "Đăng xuất"

ui/web/src/i18n/locales/zh/login.json

@@ -33,7 +33,8 @@
     "gatewayToken": "Gateway Token",
     "tokenPlaceholder": "Bearer token",
     "userId": "用户 ID",
-    "userIdPlaceholder": "您的用户 ID"
+    "userIdPlaceholder": "您的用户 ID",
+    "userIdHint": "使用 \"system\" 获取完整系统权限"
   },
   "noAccessHint": "请联系管理员将您添加到租户，或使用其他账号登录。",
   "logout": "退出登录"

ui/web/src/pages/login/token-form.tsx

@@ -8,7 +8,7 @@ interface TokenFormProps {
 
 export function TokenForm({ onSubmit }: TokenFormProps) {
   const { t } = useTranslation("login");
-  const [userId, setUserId] = useState("");
+  const [userId, setUserId] = useState("system");
   const [token, setToken] = useState("");
   const [connecting, setConnecting] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -63,6 +63,9 @@ export function TokenForm({ onSubmit }: TokenFormProps) {
           autoFocus
           disabled={connecting}
         />
+        <p className="text-xs text-muted-foreground">
+          {t("token.userIdHint")}
+        </p>
       </div>
 
       <div className="space-y-2">