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>

backend/internal/service/auth_service_register_test.go

@@ -182,6 +182,14 @@ func TestAuthService_Register_CheckEmailError(t *testing.T) {
 	require.ErrorIs(t, err, ErrServiceUnavailable)
 }
 
+func TestAuthService_Register_ReservedEmail(t *testing.T) {
+	repo := &userRepoStub{}
+	service := newAuthService(repo, nil, nil)
+
+	_, _, err := service.Register(context.Background(), "linuxdo-123@linuxdo-connect.invalid", "password")
+	require.ErrorIs(t, err, ErrEmailReserved)
+}
+
 func TestAuthService_Register_CreateError(t *testing.T) {
 	repo := &userRepoStub{createErr: errors.New("create failed")}
 	service := newAuthService(repo, map[string]string{