feat(03-01): create 7 core models and 3 service interfaces for Phase 3

- StorageNode, StorageScanOptions models
- SearchResult, SearchOptions models
- DuplicateItem, DuplicateGroup, DuplicateScanOptions models
- IStorageService, ISearchService, IDuplicatesService interfaces

SharepointToolbox/Core/Models/StorageNode.cs

@@ -0,0 +1,16 @@
+namespace SharepointToolbox.Core.Models;
+
+public class StorageNode
+{
+    public string Name { get; set; } = string.Empty;
+    public string Url { get; set; } = string.Empty;
+    public string SiteTitle { get; set; } = string.Empty;
+    public string Library { get; set; } = string.Empty;
+    public long TotalSizeBytes { get; set; }
+    public long FileStreamSizeBytes { get; set; }
+    public long VersionSizeBytes => Math.Max(0L, TotalSizeBytes - FileStreamSizeBytes);
+    public long TotalFileCount { get; set; }
+    public DateTime? LastModified { get; set; }
+    public int IndentLevel { get; set; }
+    public List<StorageNode> Children { get; set; } = new();
+}