fix(mobile-app): 修复短信验证码界面问题

- 减小验证码输入框字体大小从 24.sp 到 20.sp，防止在某些设备上被截断
- 添加行高 height: 1.2 确保文字垂直居中
- 在手机号注册页面添加 60 秒倒计时功能
- 倒计时期间禁用获取验证码按钮
- 显示剩余秒数提示 "X秒后重新发送"

frontend/mobile-app/lib/features/auth/presentation/pages/phone_register_page.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -35,6 +36,11 @@ class _PhoneRegisterPageState extends ConsumerState<PhoneRegisterPage> {
   bool _isSending = false;
   String? _errorMessage;
 
+  // 倒计时
+  int _countdown = 0;
+  Timer? _countdownTimer;
+  bool _canSend = true;
+
   @override
   void initState() {
     super.initState();
@@ -44,12 +50,31 @@ class _PhoneRegisterPageState extends ConsumerState<PhoneRegisterPage> {
 
   @override
   void dispose() {
+    _countdownTimer?.cancel();
     _phoneController.removeListener(_onPhoneChanged);
     _phoneController.dispose();
     _phoneFocusNode.dispose();
     super.dispose();
   }
 
+  void _startCountdown() {
+    _countdown = 60;
+    _canSend = false;
+    _countdownTimer?.cancel();
+    _countdownTimer = Timer.periodic(const Duration(seconds: 1), (timer) {
+      if (_countdown > 0) {
+        setState(() {
+          _countdown--;
+        });
+      } else {
+        setState(() {
+          _canSend = true;
+        });
+        timer.cancel();
+      }
+    });
+  }
+
   void _onPhoneChanged() {
     // 清除错误信息
     if (_errorMessage != null) {
@@ -93,6 +118,9 @@ class _PhoneRegisterPageState extends ConsumerState<PhoneRegisterPage> {
 
       if (!mounted) return;
 
+      // 启动倒计时
+      _startCountdown();
+
       // 跳转到验证码页面
       context.push(
         RoutePaths.smsVerify,
@@ -275,14 +303,15 @@ class _PhoneRegisterPageState extends ConsumerState<PhoneRegisterPage> {
 
   Widget _buildNextButton() {
     final isValid = _isValidPhoneNumber(_phoneController.text.trim());
+    final canClick = isValid && !_isSending && _canSend;
 
     return GestureDetector(
-      onTap: isValid && !_isSending ? _sendSmsCode : null,
+      onTap: canClick ? _sendSmsCode : null,
       child: Container(
         width: double.infinity,
         padding: EdgeInsets.symmetric(vertical: 16.h),
         decoration: BoxDecoration(
-          color: isValid && !_isSending
+          color: canClick
               ? const Color(0xFF2E7D32)
               : const Color(0xFFE0E0E0),
           borderRadius: BorderRadius.circular(12.r),
@@ -298,11 +327,11 @@ class _PhoneRegisterPageState extends ConsumerState<PhoneRegisterPage> {
                   ),
                 )
               : Text(
-                  '获取验证码',
+                  !_canSend ? '$_countdown秒后重新发送' : '获取验证码',
                   style: TextStyle(
                     fontSize: 16.sp,
                     fontWeight: FontWeight.w600,
-                    color: isValid ? Colors.white : const Color(0xFF999999),
+                    color: canClick ? Colors.white : const Color(0xFF999999),
                   ),
                 ),
         ),

frontend/mobile-app/lib/features/auth/presentation/pages/sms_verify_page.dart

@@ -404,9 +404,10 @@ class _SmsVerifyPageState extends ConsumerState<SmsVerifyPage> {
                 ),
               ),
               style: TextStyle(
-                fontSize: 24.sp,
+                fontSize: 20.sp,
                 fontWeight: FontWeight.w700,
                 color: const Color(0xFF333333),
+                height: 1.2,
               ),
               onChanged: (value) => _onCodeChanged(index, value),
             ),