refactor: 移除 SAML/LDAP/MFA 登录方式，仅保留本地账号密码和 OIDC

后端移除：
- SAML: auth/saml.go, handler/saml_handler.go, account_saml_settings_handler.go,
  model/account_saml_settings.go, model/saml_idp_config.go, repo/*.go
- LDAP: auth/ldap.go, handler/ldap_handler.go, model/account_ldap_settings.go,
  repo/account_ldap_settings_repo.go
- MFA: auth/mfa.go, handler/mfa_handler.go
- auth_service: 移除 mfaService 依赖、MFARequired 字段、LoginWithMFA 方法
- auth_handler: 移除 LoginMFA handler、MFA 分支逻辑
- bootstrap: 移除 SAML/LDAP/MFA service 初始化和 handler 注册
- sso_middleware: 精简为仅支持 OIDC provider
- router: 移除 SAML/LDAP/MFA 路由注册
- config: 移除 SAMLConfig/LDAPConfig struct 和 defaults

前端移除：
- v3/login: 移除 MFA 验证流程和 SAML 登录入口
- v3/api/auth: 移除 MFA 响应处理
- v3/routes: 移除 SSO login 路由
- dashboard: 移除 MFA 设置页面、SAML 安全设置页面
- i18n: 移除 mfa.json
- featureFlags: 移除 SAML feature flag

.env.example / .env: 移除 SAML/LDAP 配置段

frontend/app/javascript/v3/api/auth.js

@@ -12,38 +12,15 @@ export const login = async ({
   ssoConversationId,
   ...credentials
 }) => {
-  try {
-    const response = await wootAPI.post('auth/sign_in', credentials);
-
-    // Check if MFA is required
-    if (response.status === 206 && response.data.mfa_required) {
-      // Return MFA data instead of throwing error
-      return {
-        mfaRequired: true,
-        mfaToken: response.data.mfa_token,
-      };
-    }
-
-    setAuthCredentials(response);
-    clearLocalStorageOnLogout();
-    window.location = getLoginRedirectURL({
-      ssoAccountId,
-      ssoConversationId,
-      user: response.data.data,
-    });
-    return null;
-  } catch (error) {
-    // Check if it's an MFA required response
-    if (error.response?.status === 206 && error.response?.data?.mfa_required) {
-      return {
-        mfaRequired: true,
-        mfaToken: error.response.data.mfa_token,
-      };
-    }
-    const loginError = new Error(parseAPIErrorResponse(error));
-    loginError.errorCode = error.response?.data?.error_code;
-    throw loginError;
-  }
+  const response = await wootAPI.post('auth/sign_in', credentials);
+  setAuthCredentials(response);
+  clearLocalStorageOnLogout();
+  window.location = getLoginRedirectURL({
+    ssoAccountId,
+    ssoConversationId,
+    user: response.data.data,
+  });
+  return null;
 };
 
 export const resendConfirmation = async ({ email, hCaptchaClientResponse }) => {