feat(02-03): implement ISiteListService and SiteListService with admin URL derivation

- SiteInfo record added to Core/Models
- ISiteListService interface with GetSitesAsync signature
- SiteListService derives admin URL via Regex, connects via SessionManager
- Filters to Active sites only, excludes OneDrive personal (-my.sharepoint.com)
- Access denied ServerException wrapped as InvalidOperationException with actionable message
- DeriveAdminUrl marked internal static for unit testability
- InternalsVisibleTo added to AssemblyInfo.cs to expose internal to test project
- 2 DeriveAdminUrl tests pass; full suite: 53 pass, 4 skip, 0 fail

SharepointToolbox/AssemblyInfo.cs

@@ -0,0 +1,13 @@
+using System.Runtime.CompilerServices;
+using System.Windows;
+
+[assembly: InternalsVisibleTo("SharepointToolbox.Tests")]
+
+[assembly:ThemeInfo(
+    ResourceDictionaryLocation.None,            //where theme specific resource dictionaries are located
+                                                //(used if a resource is not found in the page,
+                                                // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly   //where the generic resource dictionary is located
+                                                //(used if a resource is not found in the page,
+                                                // app, or any theme specific resource dictionaries)
+)]