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/keys/UseKeyModal.vue

@@ -864,8 +864,8 @@ function generateOpenCodeConfig(platform: string, baseUrl: string, apiKey: strin
         }
       }
     },
-    'gemini-3-pro-image': {
-      name: 'Gemini 3 Pro (Image)',
+    'gemini-3.1-flash-image': {
+      name: 'Gemini 3.1 Flash Image',
       limit: {
         context: 1048576,
         output: 65536