Backend:
- New POST /api/image-studio/save-to-library endpoint
Saves generated base64 images to workspace disk and creates ContentAsset
record for the unified asset library. Returns asset_id, file_url, filename.
Frontend:
- Added saveImageToLibrary() to useImageStudio hook
- CreateStudio auto-saves generated images to asset library after creation
- All 8 API operations now use _formatErrorMessage() helper
for 402/429 subscription limit errors with upgrade prompts
instead of generic error messages
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