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/DuplicateScanOptions.cs

@@ -0,0 +1,12 @@
+namespace SharepointToolbox.Core.Models;
+
+public record DuplicateScanOptions(
+    string Mode = "Files",          // "Files" or "Folders"
+    bool MatchSize = true,
+    bool MatchCreated = false,
+    bool MatchModified = false,
+    bool MatchSubfolderCount = false,
+    bool MatchFileCount = false,
+    bool IncludeSubsites = false,
+    string? Library = null
+);