feat(19-01): add AppRegistrationService with rollback, model, and interface

- AppRegistrationResult discriminated result (Success/Failure/FallbackRequired)
- TenantProfile.AppId nullable string for storing registered app ID
- IAppRegistrationService interface (IsGlobalAdminAsync, RegisterAsync, RemoveAsync, ClearMsalSessionAsync)
- AppRegistrationService: sequential registration with rollback, transitiveMemberOf admin check, MSAL eviction

SharepointToolbox/Core/Models/TenantProfile.cs

@@ -6,4 +6,11 @@ public class TenantProfile
     public string TenantUrl { get; set; } = string.Empty;
     public string ClientId { get; set; } = string.Empty;
     public LogoData? ClientLogo { get; set; }
+
+    /// <summary>
+    /// The Azure AD application (client) ID registered for this tenant profile.
+    /// Null when no app registration has been performed yet.
+    /// Cleared to null after successful removal.
+    /// </summary>
+    public string? AppId { get; set; }
 }