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/migrations/060_add_gemini31_flash_image_to_model_mapping.sql

@@ -0,0 +1,46 @@
+-- Add gemini-3.1-flash-image and gemini-3.1-flash-image-preview to model_mapping
+--
+-- Background:
+-- Antigravity now supports gemini-3.1-flash-image as the latest image generation model,
+-- replacing the previous gemini-3-pro-image.
+--
+-- Strategy:
+-- Directly overwrite the entire model_mapping with updated mappings
+-- This ensures consistency with DefaultAntigravityModelMapping in constants.go
+
+UPDATE accounts
+SET credentials = jsonb_set(
+    credentials,
+    '{model_mapping}',
+    '{
+        "claude-opus-4-6-thinking": "claude-opus-4-6-thinking",
+        "claude-opus-4-6": "claude-opus-4-6-thinking",
+        "claude-opus-4-5-thinking": "claude-opus-4-6-thinking",
+        "claude-opus-4-5-20251101": "claude-opus-4-6-thinking",
+        "claude-sonnet-4-6": "claude-sonnet-4-6",
+        "claude-sonnet-4-5": "claude-sonnet-4-5",
+        "claude-sonnet-4-5-thinking": "claude-sonnet-4-5-thinking",
+        "claude-sonnet-4-5-20250929": "claude-sonnet-4-5",
+        "claude-haiku-4-5": "claude-sonnet-4-5",
+        "claude-haiku-4-5-20251001": "claude-sonnet-4-5",
+        "gemini-2.5-flash": "gemini-2.5-flash",
+        "gemini-2.5-flash-lite": "gemini-2.5-flash-lite",
+        "gemini-2.5-flash-thinking": "gemini-2.5-flash-thinking",
+        "gemini-2.5-pro": "gemini-2.5-pro",
+        "gemini-3-flash": "gemini-3-flash",
+        "gemini-3-pro-high": "gemini-3-pro-high",
+        "gemini-3-pro-low": "gemini-3-pro-low",
+        "gemini-3-flash-preview": "gemini-3-flash",
+        "gemini-3-pro-preview": "gemini-3-pro-high",
+        "gemini-3.1-pro-high": "gemini-3.1-pro-high",
+        "gemini-3.1-pro-low": "gemini-3.1-pro-low",
+        "gemini-3.1-pro-preview": "gemini-3.1-pro-high",
+        "gemini-3.1-flash-image": "gemini-3.1-flash-image",
+        "gemini-3.1-flash-image-preview": "gemini-3.1-flash-image",
+        "gpt-oss-120b-medium": "gpt-oss-120b-medium",
+        "tab_flash_lite_preview": "tab_flash_lite_preview"
+    }'::jsonb
+)
+WHERE platform = 'antigravity'
+  AND deleted_at IS NULL
+  AND credentials->'model_mapping' IS NOT NULL;