fix(password): 统一登录密码和支付密码状态查询为服务端 API

问题：
- isPasswordSet() 读 SecureStorage，重装/换设备后丢失 → 页面误判未设密码
- isPaymentPasswordSet() 读 response.data['isSet']，但实际格式为
  { success, data: { isSet } }，取到 null → 始终返回 false

修复：
- 后端新增 GET /user/password-status 接口（isLoginPasswordSet 方法）
- 前端 isPasswordSet() 改为调用服务端 API
- 两个方法均使用正确的 response.data['data']['isSet'] 解包

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

backend/services/identity-service/src/api/controllers/user-account.controller.ts

@@ -756,6 +756,17 @@ export class UserAccountController {
     return { valid };
   }
 
+  // ============ 密码状态查询 ============
+
+  @Get('password-status')
+  @ApiBearerAuth()
+  @ApiOperation({ summary: '查询登录密码状态', description: '查询当前用户是否已设置登录密码' })
+  @ApiResponse({ status: 200, description: '{ isSet: boolean }' })
+  async getPasswordStatus(@CurrentUser() user: CurrentUserData) {
+    const isSet = await this.userService.isLoginPasswordSet(user.userId);
+    return { isSet };
+  }
+
   // ============ 支付密码相关 ============
 
   @Get('payment-password-status')

backend/services/identity-service/src/application/services/user-application.service.ts

@@ -2861,6 +2861,14 @@ export class UserApplicationService {
   /**
    * 查询是否已设置支付密码
    */
+  async isLoginPasswordSet(userId: string): Promise<boolean> {
+    const user = await this.prisma.userAccount.findUnique({
+      where: { userId: BigInt(userId) },
+      select: { passwordHash: true },
+    });
+    return !!user?.passwordHash;
+  }
+
   async isPaymentPasswordSet(userId: string): Promise<boolean> {
     const user = await this.prisma.userAccount.findUnique({
       where: { userId: BigInt(userId) },

frontend/mobile-app/lib/core/services/account_service.dart

@@ -2059,11 +2059,16 @@ class AccountService {
 
   /// 检查是否已设置密码
   Future<bool> isPasswordSet() async {
-    debugPrint('$_tag isPasswordSet() - 检查是否已设置密码');
-    final isSet = await _secureStorage.read(key: StorageKeys.isPasswordSet);
-    final result = isSet == 'true';
+    debugPrint('$_tag isPasswordSet() - 查询登录密码状态');
+    try {
+      final response = await _apiClient.get('/user/password-status');
+      final result = response.data['data']['isSet'] == true;
       debugPrint('$_tag isPasswordSet() - 结果: $result');
       return result;
+    } catch (e) {
+      debugPrint('$_tag isPasswordSet() - 异常: $e');
+      rethrow;
+    }
   }
 
   /// 修改登录密码