feat(auth): 优化登录页布局与字段反馈

lib/pages/auth/auth_logic.dart

@@ -8,29 +8,44 @@ class AuthLogic {
   String password = '';
   bool busy = false;
   String? errorMessage;
+  AuthFieldError? fieldError;
 
   AuthLogic({this.signUpMode = false, this.errorMessage});
 
   void toggleMode() {
     signUpMode = !signUpMode;
     errorMessage = null;
+    fieldError = null;
   }
 
   /// 返回 null 表示校验通过，否则返回错误文案（由视图用 l10n 映射）
   AuthFieldError? validate() {
     final mail = email.trim();
-    if (mail.isEmpty) return AuthFieldError.emailRequired;
-    if (!_emailPattern.hasMatch(mail)) return AuthFieldError.emailInvalid;
-    if (password.isEmpty) return AuthFieldError.passwordRequired;
+    if (mail.isEmpty) return _record(AuthFieldError.emailRequired);
+    if (!_emailPattern.hasMatch(mail)) {
+      return _record(AuthFieldError.emailInvalid);
+    }
+    if (password.isEmpty) return _record(AuthFieldError.passwordRequired);
     if (password.length < 8 || password.length > 64) {
-      return AuthFieldError.passwordLength;
+      return _record(AuthFieldError.passwordLength);
     }
     if (!_hasLetter.hasMatch(password) || !_hasDigit.hasMatch(password)) {
-      return AuthFieldError.passwordWeak;
+      return _record(AuthFieldError.passwordWeak);
     }
+    fieldError = null;
     return null;
   }
 
+  /// 清除当前字段错误；输入变化时由页面调用。
+  void clearFieldError() {
+    fieldError = null;
+  }
+
+  AuthFieldError _record(AuthFieldError error) {
+    fieldError = error;
+    return error;
+  }
+
   String get normalizedEmail => email.trim().toLowerCase();
 
   static final _emailPattern = RegExp(