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/Models/TenantProfile.cs

@@ -0,0 +1,8 @@
+namespace SharepointToolbox.Core.Models;
+
+public class TenantProfile
+{
+    public string Name { get; set; } = string.Empty;
+    public string TenantUrl { get; set; } = string.Empty;
+    public string ClientId { get; set; } = string.Empty;
+}