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/i18n/locales/en.ts

@@ -1133,7 +1133,7 @@ export default {
       },
       imagePricing: {
         title: 'Image Generation Pricing',
-        description: 'Configure pricing for gemini-3-pro-image model. Leave empty to use default prices.'
+        description: 'Configure pricing for image generation models. Leave empty to use default prices.'
       },
       soraPricing: {
         title: 'Sora Per-Request Pricing',
@@ -2047,7 +2047,7 @@ export default {
         geminiFlashDaily: 'Flash',
         gemini3Pro: 'G3P',
         gemini3Flash: 'G3F',
-        gemini3Image: 'G3I',
+        gemini3Image: 'GImage',
         claude: 'Claude'
       },
       tier: {