fix(image-studio): add asset library saving + 402 subscription error handling

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

backend/routers/image_studio/__init__.py

@@ -15,6 +15,7 @@ from .create import router as create_router
 from .transform import router as transform_router
 from .compress import router as compress_router
 from .convert import router as convert_router
+from .save import router as save_router
 
 router = APIRouter(prefix="/api/image-studio", tags=["image-studio"])
 
@@ -28,5 +29,6 @@ router.include_router(create_router)
 router.include_router(transform_router)
 router.include_router(compress_router)
 router.include_router(convert_router)
+router.include_router(save_router)
 
 __all__ = ["router"]