feat: consolidate checkpoint metadata handling

- Extract checkpoint entry from multiple metadata locations using helper method
- Sanitize checkpoint metadata by removing transient/local-only fields
- Remove checkpoint duplication from generation parameters to store only at top level
- Update frontend to properly populate checkpoint metadata during import
- Add tests for new checkpoint handling functionality

This ensures consistent checkpoint metadata structure and prevents data duplication across different storage locations.

static/js/managers/import/DownloadManager.js

@@ -56,6 +56,15 @@ export class DownloadManager {
                     gen_params: this.importManager.recipeData.gen_params || {},
                     raw_metadata: this.importManager.recipeData.raw_metadata || {}
                 };
+
+                const checkpointMetadata =
+                    this.importManager.recipeData.checkpoint ||
+                    this.importManager.recipeData.model ||
+                    (this.importManager.recipeData.gen_params || {}).checkpoint;
+
+                if (checkpointMetadata && typeof checkpointMetadata === 'object') {
+                    completeMetadata.checkpoint = checkpointMetadata;
+                }
                 
                 // Add source_path to metadata to track where the recipe was imported from
                 if (this.importManager.importMode === 'url') {