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",