feat(auth): 优化登录页布局与字段反馈

lib/pages/auth/auth_widgets.dart

@@ -19,12 +19,14 @@ class _AuthFormStatus {
     required this.signUpMode,
     required this.busy,
     required this.errorMessage,
+    required this.fieldError,
     required this.obscurePassword,
   });
 
   final bool signUpMode;
   final bool busy;
   final String? errorMessage;
+  final AuthFieldError? fieldError;
   final bool obscurePassword;
 }
 
@@ -100,7 +102,7 @@ class _AuthCard extends StatelessWidget {
           ),
           const SizedBox(height: 6),
           Text(
-            l10n.authSubtitle,
+            status.signUpMode ? l10n.authSignUpSubtitle : l10n.authSubtitle,
             style: const TextStyle(
               color: AppColors.muted,
               fontSize: AppDimens.body,