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/features/login/view/login_page.dart

@@ -5,6 +5,7 @@ import 'package:im_app/features/login/presentation/login_state.dart';
 import 'package:im_app/features/login/presentation/login_view_model.dart';
 import 'package:im_app/features/login/view/widgets/login_otp_step.dart';
 import 'package:im_app/features/login/view/widgets/login_phone_step.dart';
+import 'package:im_app/features/login/view/widgets/login_secondary_passcode_step.dart';
 
 /// 登录页 — 两步流程：手机号 → 验证码
 ///
@@ -24,11 +25,13 @@ class _LoginPageState extends ConsumerState<LoginPage> {
   // demo 预填，上线前去掉
   final _phoneCtrl = TextEditingController(text: '83465308');
   final _otpCtrl = TextEditingController(text: '0000');
+  final _passcodeCtrl = TextEditingController();
 
   @override
   void dispose() {
     _phoneCtrl.dispose();
     _otpCtrl.dispose();
+    _passcodeCtrl.dispose();
     super.dispose();
   }
 
@@ -44,8 +47,15 @@ class _LoginPageState extends ConsumerState<LoginPage> {
         .verifyAndLogin(_otpCtrl.text.trim());
   }
 
+  void _verifyPasscode() {
+    ref
+        .read(loginViewModelProvider.notifier)
+        .verifyPasscode(_passcodeCtrl.text);
+  }
+
   void _backToPhone() {
     _otpCtrl.clear();
+    _passcodeCtrl.clear();
     ref.read(loginViewModelProvider.notifier).backToPhone();
   }
 
@@ -53,22 +63,35 @@ class _LoginPageState extends ConsumerState<LoginPage> {
   Widget build(BuildContext context) {
     final state = ref.watch(loginViewModelProvider);
 
+    Widget body;
+    switch (state.step) {
+      case LoginStep.phone:
+        body = LoginPhoneStep(
+          phoneCtrl: _phoneCtrl,
+          state: state,
+          onSendOtp: () => _sendOtp(state),
+        );
+      case LoginStep.otp:
+        body = LoginOtpStep(
+          otpCtrl: _otpCtrl,
+          state: state,
+          onVerifyAndLogin: _verifyAndLogin,
+          onBackToPhone: _backToPhone,
+        );
+      case LoginStep.secondaryPasscode:
+        body = LoginSecondaryPasscodeStep(
+          passcodeCtrl: _passcodeCtrl,
+          state: state,
+          onVerifyPasscode: _verifyPasscode,
+          onBackToPhone: _backToPhone,
+        );
+    }
+
     return Scaffold(
       appBar: AppBar(automaticallyImplyLeading: false, title: const Text('登录')),
       body: Padding(
         padding: const EdgeInsets.symmetric(horizontal: 32),
-        child: state.step == LoginStep.phone
-            ? LoginPhoneStep(
-                phoneCtrl: _phoneCtrl,
-                state: state,
-                onSendOtp: () => _sendOtp(state),
-              )
-            : LoginOtpStep(
-                otpCtrl: _otpCtrl,
-                state: state,
-                onVerifyAndLogin: _verifyAndLogin,
-                onBackToPhone: _backToPhone,
-              ),
+        child: SingleChildScrollView(child: body),
       ),
     );
   }