feat: replace gemini-3-pro-image with gemini-3.1-flash-image

- Add migration 060 to update model_mapping for all antigravity accounts
- Remove gemini-3-pro-image and gemini-3-pro-image-preview mappings
- Add gemini-3.1-flash-image and gemini-3.1-flash-image-preview mappings
- Update frontend usage window to show GImage for new model
- Update isImageGenerationModel to support new model

backend/internal/domain/constants.go

@@ -92,16 +92,18 @@ var DefaultAntigravityModelMapping = map[string]string{
 	"gemini-3-flash":     "gemini-3-flash",
 	"gemini-3-pro-high":  "gemini-3-pro-high",
 	"gemini-3-pro-low":   "gemini-3-pro-low",
-	"gemini-3-pro-image": "gemini-3-pro-image",
 	// Gemini 3 preview 映射
 	"gemini-3-flash-preview":     "gemini-3-flash",
 	"gemini-3-pro-preview":       "gemini-3-pro-high",
-	"gemini-3-pro-image-preview": "gemini-3-pro-image",
 	// Gemini 3.1 白名单
 	"gemini-3.1-pro-high": "gemini-3.1-pro-high",
 	"gemini-3.1-pro-low":  "gemini-3.1-pro-low",
 	// Gemini 3.1 preview 映射
 	"gemini-3.1-pro-preview": "gemini-3.1-pro-high",
+	// Gemini 3.1 image 白名单
+	"gemini-3.1-flash-image": "gemini-3.1-flash-image",
+	// Gemini 3.1 image preview 映射
+	"gemini-3.1-flash-image-preview": "gemini-3.1-flash-image",
 	// 其他官方模型
 	"gpt-oss-120b-medium":    "gpt-oss-120b-medium",
 	"tab_flash_lite_preview": "tab_flash_lite_preview",

backend/internal/service/antigravity_gateway_service.go

@@ -3757,14 +3757,17 @@ func (s *AntigravityGatewayService) extractImageSize(body []byte) string {
 }
 
 // isImageGenerationModel 判断模型是否为图片生成模型
-// 支持的模型：gemini-3-pro-image, gemini-3-pro-image-preview, gemini-2.5-flash-image 等
+// 支持的模型：gemini-3.1-flash-image, gemini-3-pro-image, gemini-2.5-flash-image 等
 func isImageGenerationModel(model string) bool {
 	modelLower := strings.ToLower(model)
 	// 移除 models/ 前缀
 	modelLower = strings.TrimPrefix(modelLower, "models/")
 
 	// 精确匹配或前缀匹配
-	return modelLower == "gemini-3-pro-image" ||
+	return modelLower == "gemini-3.1-flash-image" ||
+		modelLower == "gemini-3.1-flash-image-preview" ||
+		strings.HasPrefix(modelLower, "gemini-3.1-flash-image-") ||
+		modelLower == "gemini-3-pro-image" ||
 		modelLower == "gemini-3-pro-image-preview" ||
 		strings.HasPrefix(modelLower, "gemini-3-pro-image-") ||
 		modelLower == "gemini-2.5-flash-image" ||

backend/internal/service/gateway_service.go

@@ -470,7 +470,7 @@ type ForwardResult struct {
 	FirstTokenMs     *int // 首字时间（流式请求）
 	ClientDisconnect bool // 客户端是否在流式传输过程中断开
 
-	// 图片生成计费字段（仅 gemini-3-pro-image 使用）
+	// 图片生成计费字段（图片生成模型使用）
 	ImageCount int    // 生成的图片数量
 	ImageSize  string // 图片尺寸 "1K", "2K", "4K"