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

frontend/src/composables/useModelWhitelist.ts

@@ -91,10 +91,10 @@ const antigravityModels = [
   'gemini-3-flash',
   'gemini-3-pro-high',
   'gemini-3-pro-low',
-  'gemini-3-pro-image',
   // Gemini 3.1 系列
   'gemini-3.1-pro-high',
   'gemini-3.1-pro-low',
+  'gemini-3.1-flash-image',
   // 其他
   'gpt-oss-120b-medium',
   'tab_flash_lite_preview'