fix(kyc): 修复更换手机号页面发送验证码按钮可重复点击的问题

- 添加 _isSendingCode 状态检查，防止发送中重复点击
- 发送中显示 loading 指示器
- 倒计时期间和发送中均禁用按钮

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/kyc/presentation/pages/change_phone_page.dart

@@ -587,9 +587,10 @@ class _ChangePhonePageState extends ConsumerState<ChangePhonePage> {
   }
 
   Widget _buildSendCodeButton(Future<void> Function() onSend) {
+    final canSend = _countdown == 0 && !_isSendingCode;
     return Center(
       child: GestureDetector(
-        onTap: _countdown > 0 ? null : () async {
+        onTap: canSend ? () async {
           try {
             await onSend();
           } catch (e) {
@@ -599,15 +600,24 @@ class _ChangePhonePageState extends ConsumerState<ChangePhonePage> {
               });
             }
           }
-        },
-        child: Text(
-          _countdown > 0 ? '$_countdown秒后重新发送' : '发送验证码',
-          style: TextStyle(
-            fontSize: 14.sp,
-            fontWeight: FontWeight.w500,
-            color: _countdown > 0 ? const Color(0xFF999999) : const Color(0xFF2E7D32),
-          ),
-        ),
+        } : null,
+        child: _isSendingCode
+            ? SizedBox(
+                width: 20.sp,
+                height: 20.sp,
+                child: const CircularProgressIndicator(
+                  strokeWidth: 2,
+                  valueColor: AlwaysStoppedAnimation<Color>(Color(0xFF999999)),
+                ),
+              )
+            : Text(
+                _countdown > 0 ? '$_countdown秒后重新发送' : '发送验证码',
+                style: TextStyle(
+                  fontSize: 14.sp,
+                  fontWeight: FontWeight.w500,
+                  color: canSend ? const Color(0xFF2E7D32) : const Color(0xFF999999),
+                ),
+              ),
       ),
     );
   }