Fix SSH password authentication logic by removing requirePassword field

This commit eliminates the confusing requirePassword field that was causing
authentication issues where users couldn't disable password requirements.

Changes:
- Remove requirePassword field from database schema and migrations
- Simplify SSH authentication logic by removing special case branches
- Update frontend to remove requirePassword UI controls
- Clean up translation files to remove unused strings
- Support standard SSH empty password authentication

The new design follows the principle of "good taste" - password field itself
now expresses the requirement: null/empty = no password auth, value = use password.

Fixes the issue where setting requirePassword=false didn't work as expected.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/locales/zh/translation.json

@@ -576,8 +576,6 @@
     "upload": "上传",
     "authentication": "认证方式",
     "password": "密码",
-    "requirePassword": "要求密码",
-    "requirePasswordDescription": "禁用时，可以在不输入密码的情况下保存会话",
     "key": "密钥",
     "credential": "凭证",
     "selectCredential": "选择凭证",