Add support for passwordless host authentication

Allow adding SSH hosts without authentication credentials by introducing
a new "none" auth type. This enables users to configure hosts for later
use with SSH agent or manual credential addition.

Changes:
- Add authType "none" to type definitions
- Update frontend form to support "None" authentication option
- Skip credential validation for "none" auth type
- Update backend to accept hosts without credentials
- Add i18n support for English and Chinese

Fixes #278

src/locales/zh/translation.json

@@ -635,6 +635,8 @@
     "password": "密码",
     "key": "密钥",
     "credential": "凭证",
+    "none": "无",
+    "noneDescription": "无需认证凭证。您可以稍后添加凭证或使用 SSH 代理进行认证。",
     "selectCredential": "选择凭证",
     "selectCredentialPlaceholder": "选择一个凭证...",
     "credentialRequired": "使用凭证认证时需要选择凭证",