fix(frontend): 添加新账号时显示加载状态提示

- 添加 _isAddingAccount 状态变量
- 点击"添加新账号"时显示"正在准备..."加载提示
- 添加 try-catch 错误处理，失败时显示错误提示
- 防止重复点击

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

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

frontend/mobile-app/lib/features/account/presentation/pages/account_switch_page.dart

@@ -20,6 +20,7 @@ class _AccountSwitchPageState extends ConsumerState<AccountSwitchPage> {
   String? _currentAccountId;
   bool _isLoading = true;
   bool _isSwitching = false;
+  bool _isAddingAccount = false;
 
   @override
   void initState() {
@@ -88,16 +89,34 @@ class _AccountSwitchPageState extends ConsumerState<AccountSwitchPage> {
   }
 
   Future<void> _addNewAccount() async {
-    // 保存当前账号数据
-    final multiAccountService = ref.read(multiAccountServiceProvider);
-    await multiAccountService.saveCurrentAccountData();
+    if (_isAddingAccount) return;
 
-    // 退出当前账号但保留数据
-    await multiAccountService.logoutCurrentAccount();
+    setState(() => _isAddingAccount = true);
 
-    // 跳转到向导页创建新账号
-    if (mounted) {
-      context.go(RoutePaths.guide);
+    try {
+      final multiAccountService = ref.read(multiAccountServiceProvider);
+
+      // 保存当前账号数据
+      await multiAccountService.saveCurrentAccountData();
+
+      // 退出当前账号但保留数据
+      await multiAccountService.logoutCurrentAccount();
+
+      // 跳转到向导页创建新账号
+      if (mounted) {
+        context.go(RoutePaths.guide);
+      }
+    } catch (e) {
+      debugPrint('[AccountSwitchPage] 添加新账号失败: $e');
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          SnackBar(content: Text('添加新账号失败: $e')),
+        );
+      }
+    } finally {
+      if (mounted) {
+        setState(() => _isAddingAccount = false);
+      }
     }
   }
 
@@ -179,18 +198,18 @@ class _AccountSwitchPageState extends ConsumerState<AccountSwitchPage> {
                 color: Color(0xFFD4AF37),
               ),
             )
-          : _isSwitching
-              ? const Center(
+          : (_isSwitching || _isAddingAccount)
+              ? Center(
                   child: Column(
                     mainAxisAlignment: MainAxisAlignment.center,
                     children: [
-                      CircularProgressIndicator(
+                      const CircularProgressIndicator(
                         color: Color(0xFFD4AF37),
                       ),
-                      SizedBox(height: 16),
+                      const SizedBox(height: 16),
                       Text(
-                        '正在切换账号...',
-                        style: TextStyle(
+                        _isAddingAccount ? '正在准备...' : '正在切换账号...',
+                        style: const TextStyle(
                           fontSize: 14,
                           color: Color(0xFF5D4037),
                         ),