feat(01-02): add Core models and WeakReferenceMessenger messages

- TenantProfile (plain class, mutable, fields match JSON schema: Name/TenantUrl/ClientId)
- OperationProgress (record with Indeterminate factory, used by all feature services via IProgress<T>)
- TenantSwitchedMessage (ValueChangedMessage<TenantProfile>, broadcast-ready)
- LanguageChangedMessage (ValueChangedMessage<string>, broadcast-ready)

SharepointToolbox/Core/Messages/LanguageChangedMessage.cs

@@ -0,0 +1,8 @@
+using CommunityToolkit.Mvvm.Messaging.Messages;
+
+namespace SharepointToolbox.Core.Messages;
+
+public sealed class LanguageChangedMessage : ValueChangedMessage<string>
+{
+    public LanguageChangedMessage(string cultureCode) : base(cultureCode) { }
+}