- Add StorageService implementing IStorageService
- Load Folder.StorageMetrics, TimeLastModified, Name, ServerRelativeUrl in one CSOM round-trip per folder
- CollectStorageAsync returns one StorageNode per document library at IndentLevel=0
- With FolderDepth>0, CollectSubfoldersAsync recurses into child folders
- All CSOM calls use ExecuteQueryRetryHelper.ExecuteQueryRetryAsync (3 call sites)
- System/hidden lists skipped (Hidden=true or BaseType != DocumentLibrary)
- Forms/ and _-prefixed system folders skipped during subfolder recursion
- ct.ThrowIfCancellationRequested() called at top of every recursive step
