fix(security): 修复支付密码页面加载失败时静默降级为设置模式的 bug

加载状态失败时改为显示错误提示和重试按钮，避免误导用户认为未设置支付密码。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/mobile-app/lib/features/security/presentation/pages/change_payment_password_page.dart

@@ -31,6 +31,7 @@ class _ChangePaymentPasswordPageState
   bool _isSubmitting = false;
   bool _hasPaymentPassword = false;
   bool _isLoading = true;
+  bool _isLoadError = false;
 
   @override
   void initState() {
@@ -60,7 +61,7 @@ class _ChangePaymentPasswordPageState
     } catch (e) {
       if (mounted) {
         setState(() {
-          _hasPaymentPassword = false;
+          _isLoadError = true;
           _isLoading = false;
         });
       }
@@ -160,7 +161,24 @@ class _ChangePaymentPasswordPageState
                               Color(0xFFD4AF37)),
                         ),
                       )
-                    : SingleChildScrollView(
+                    : _isLoadError
+                        ? Center(
+                            child: Column(
+                              mainAxisSize: MainAxisSize.min,
+                              children: [
+                                const Icon(Icons.error_outline, color: Colors.red, size: 48),
+                                const SizedBox(height: 12),
+                                const Text('加载失败，请重试', style: TextStyle(color: Colors.red)),
+                                const SizedBox(height: 16),
+                                ElevatedButton(
+                                  onPressed: _loadStatus,
+                                  style: ElevatedButton.styleFrom(backgroundColor: const Color(0xFFD4AF37)),
+                                  child: const Text('重试', style: TextStyle(color: Colors.white)),
+                                ),
+                              ],
+                            ),
+                          )
+                        : SingleChildScrollView(
                         padding: const EdgeInsets.all(16),
                         child: Column(
                           crossAxisAlignment: CrossAxisAlignment.start,