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/DuplicateGroup.cs

@@ -0,0 +1,8 @@
+namespace SharepointToolbox.Core.Models;
+
+public class DuplicateGroup
+{
+    public string GroupKey { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+    public List<DuplicateItem> Items { get; set; } = new();
+}