feat(antigravity): 支持 Refresh Token 批量导入创建 OAuth 账号

后端新增 ValidateRefreshToken service 方法和 POST /oauth/refresh-token 端点，
前端新增 API/Composable/UI 集成，OAuthAuthorizationFlow i18n 动态化，
支持在 Antigravity 创建账号时批量粘贴 Refresh Token 自动验证并创建账号。

backend/internal/handler/admin/antigravity_oauth_handler.go

@@ -65,3 +65,27 @@ func (h *AntigravityOAuthHandler) ExchangeCode(c *gin.Context) {
 
 	response.Success(c, tokenInfo)
 }
+
+// AntigravityRefreshTokenRequest represents the request for validating Antigravity refresh token
+type AntigravityRefreshTokenRequest struct {
+	RefreshToken string `json:"refresh_token" binding:"required"`
+	ProxyID      *int64 `json:"proxy_id"`
+}
+
+// RefreshToken validates an Antigravity refresh token and returns full token info
+// POST /api/v1/admin/antigravity/oauth/refresh-token
+func (h *AntigravityOAuthHandler) RefreshToken(c *gin.Context) {
+	var req AntigravityRefreshTokenRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "请求无效: "+err.Error())
+		return
+	}
+
+	tokenInfo, err := h.antigravityOAuthService.ValidateRefreshToken(c.Request.Context(), req.RefreshToken, req.ProxyID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, tokenInfo)
+}