feat(auth): 添加 Linux DO Connect OAuth 登录支持

- 新增 Linux DO OAuth 配置项和环境变量支持
- 实现 OAuth 授权流程和回调处理
- 前端添加 Linux DO 登录按钮和回调页面
- 支持通过 Linux DO 账号注册/登录
- 添加相关国际化文本

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -229,6 +229,15 @@ export default {
     sendingCode: 'Sending...',
     clickToResend: 'Click to resend code',
     resendCode: 'Resend verification code',
+    linuxdo: {
+      signIn: 'Continue with Linux.do',
+      orContinue: 'or continue with email',
+      callbackTitle: 'Signing you in',
+      callbackProcessing: 'Completing login, please wait...',
+      callbackHint: 'If you are not redirected automatically, go back to the login page and try again.',
+      callbackMissingToken: 'Missing login token, please try again.',
+      backToLogin: 'Back to Login'
+    },
     oauth: {
       code: 'Code',
       state: 'State',