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/components/account/AccountStatusIndicator.vue

@@ -181,6 +181,7 @@ const formatScopeName = (scope: string): string => {
     'gemini-3.1-pro-high': 'G3PH',
     'gemini-3.1-pro-low': 'G3PL',
     'gemini-3-pro-image': 'G3PI',
+    'gemini-3.1-flash-image': 'GImage',
     // 其他
     'gpt-oss-120b-medium': 'GPT120',
     'tab_flash_lite_preview': 'TabFL',