feat(login): 二级密码登录支持（STATUS_SECONDARY_PASSCODE_ERROR #1）

## 问题
旧版 Flutter 项目在 /vcode/check 返回 30164 时展示二级密码输入界面；
新版完全缺失此路径，有二级密码的账号无法登录。

## 改动

### networks_sdk
- `networks_sdk_method_channel_datasource.dart`：executeRequest 的
  generic catch 改为 rethrow，允许 decodeResponse override 抛出
  自定义业务异常（原为 ApiError.unknown 包裹导致数据丢失）

### 数据层
- `errors.dart`：新增 `secondaryPasscodeRequired = 30164`
- `exceptions.dart`（新增）：`SecondaryPasscodeRequiredException`
  携带 vcodeToken / recoveryEmail / hint / resetStatus
- `verify_otp_request.dart`：override decodeResponse，拦截 30164，
  从响应 data 提取字段，throw SecondaryPasscodeRequiredException
- `login_request.dart`：新增可选 password 字段 + toJson override
  （条件序列化，null 时不带 password 字段）
- `auth_repository.dart`：新增 loginWithPasscode() 接口
- `auth_repository_impl.dart`：实现 loginWithPasscode()

### 业务层
- `login_usecase.dart`：新增 loginWithSecondaryPasscode()
  （MD5 哈希 passcode → 调 AuthRepository.loginWithPasscode）
- `pubspec.yaml`：新增 crypto: ^3.0.6（用于 MD5）

### UI 层
- `login_state.dart`：新增 LoginStep.secondaryPasscode
  + vcodeToken / passcodeHint / recoveryEmail 字段
- `login_view_model.dart`：verifyAndLogin 捕获 SecondaryPasscodeRequiredException
  跳转步骤 3；新增 verifyPasscode()
- `login_secondary_passcode_step.dart`（新增）：密码输入 UI（hint 显示、
  obscured 输入框、错误提示、忘记密码占位）
- `login_page.dart`：switch 路由接入 LoginStep.secondaryPasscode

apps/im_app/lib/data/remote/verify_otp_request.dart

@@ -1,7 +1,10 @@
+import 'package:dio/dio.dart';
 import 'package:json_annotation/json_annotation.dart';
 import 'package:networks_sdk/networks_sdk.dart';
 
 import 'package:im_app/core/foundation/api_paths.dart';
+import 'package:im_app/core/foundation/errors.dart';
+import 'package:im_app/core/foundation/exceptions.dart';
 
 part 'verify_otp_request.g.dart';
 
@@ -61,4 +64,29 @@ class VerifyOtpRequest extends ApiRequestable<VerifyOtpResponse>
     this.email = '',
     this.type = 1,
   });
+
+  /// 拦截二级密码错误码 30164，将服务端 data 中的 vcode_token 等字段
+  /// 包装为 [SecondaryPasscodeRequiredException] 抛出，供上层导航至
+  /// 二级密码输入界面。其余情况委托给基类处理。
+  @override
+  VerifyOtpResponse? decodeResponse(Response response) {
+    if (response.data is Map<String, dynamic>) {
+      final json = response.data as Map<String, dynamic>;
+      final rawCode = json['code'];
+      final code = rawCode is int
+          ? rawCode
+          : int.tryParse(rawCode?.toString() ?? '') ?? 0;
+
+      if (code == ApiErrorCodes.secondaryPasscodeRequired) {
+        final data = json['data'] as Map<String, dynamic>?;
+        throw SecondaryPasscodeRequiredException(
+          vcodeToken: data?['vcode_token'] as String? ?? '',
+          recoveryEmail: data?['recovery_email'] as String? ?? '',
+          hint: data?['hint'] as String? ?? '',
+          resetStatus: data?['reset_status'] as bool? ?? false,
+        );
+      }
+    }
+    return super.decodeResponse(response);
+  }
 }