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

frontend/src/components/ImageStudio/CreateStudio.tsx

@@ -191,6 +191,7 @@ export const CreateStudio: React.FC<CreateStudioProps> = ({ onImageGenerated })
     estimateCost,
     loadTemplates,
     loadProviders,
+    saveImageToLibrary,
   } = useImageStudio();
 
   // Load meta data on mount
@@ -317,6 +318,23 @@ export const CreateStudio: React.FC<CreateStudioProps> = ({ onImageGenerated })
         seed: providerForAdvanced ? parsedSeed : undefined,
       });
 
+      // Auto-save generated images to asset library
+      if (result?.results?.length) {
+        const resolvedProvider = result.request?.provider || provider;
+        const resolvedModel = result.request?.model || effectiveModel;
+        for (const imgResult of result.results) {
+          if (imgResult.image_base64) {
+            saveImageToLibrary({
+              imageBase64: imgResult.image_base64,
+              prompt,
+              provider: resolvedProvider,
+              model: resolvedModel,
+              operation: 'image-generation',
+            }).catch((e) => console.warn('Asset library save skipped:', e.message || e));
+          }
+        }
+      }
+
       if (onImageGenerated && (result?.results?.length ?? 0) > 0) {
         onImageGenerated(result);
       }