feat(12-01): add Base64ToImageSourceConverter, localization keys, and ClientLogoPreview property

- Base64ToImageSourceConverter converts data URI strings to BitmapImage with null-safe error handling
- Registered converter in App.xaml as Base64ToImageConverter global resource
- Added 9 localization keys (EN+FR) for logo UI labels in Settings and Profile dialogs
- Added ClientLogoPreview string property to ProfileManagementViewModel with FormatLogoPreview helper
- Updated OnSelectedProfileChanged, BrowseClientLogoAsync, ClearClientLogoAsync, AutoPullClientLogoAsync
- 17 tests pass (6 converter + 11 profile VM logo tests)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SharepointToolbox/ViewModels/ProfileManagementViewModel.cs

@@ -32,6 +32,13 @@ public partial class ProfileManagementViewModel : ObservableObject
     [ObservableProperty]
     private string _validationMessage = string.Empty;
 
+    private string? _clientLogoPreview;
+    public string? ClientLogoPreview
+    {
+        get => _clientLogoPreview;
+        private set { _clientLogoPreview = value; OnPropertyChanged(); }
+    }
+
     public ObservableCollection<TenantProfile> Profiles { get; } = new();
 
     public IAsyncRelayCommand AddCommand { get; }
@@ -81,6 +88,7 @@ public partial class ProfileManagementViewModel : ObservableObject
 
     partial void OnSelectedProfileChanged(TenantProfile? value)
     {
+        ClientLogoPreview = FormatLogoPreview(value?.ClientLogo);
         BrowseClientLogoCommand.NotifyCanExecuteChanged();
         ClearClientLogoCommand.NotifyCanExecuteChanged();
         AutoPullClientLogoCommand.NotifyCanExecuteChanged();
@@ -88,6 +96,9 @@ public partial class ProfileManagementViewModel : ObservableObject
         DeleteCommand.NotifyCanExecuteChanged();
     }
 
+    private static string? FormatLogoPreview(LogoData? logo)
+        => logo is not null ? $"data:{logo.MimeType};base64,{logo.Base64}" : null;
+
     private void NotifyCommandsCanExecuteChanged()
     {
         AddCommand.NotifyCanExecuteChanged();
@@ -172,6 +183,7 @@ public partial class ProfileManagementViewModel : ObservableObject
         {
             var logo = await _brandingService.ImportLogoAsync(dialog.FileName);
             SelectedProfile.ClientLogo = logo;
+            ClientLogoPreview = FormatLogoPreview(logo);
             await _profileService.UpdateProfileAsync(SelectedProfile);
             ValidationMessage = string.Empty;
         }
@@ -188,6 +200,7 @@ public partial class ProfileManagementViewModel : ObservableObject
         try
         {
             SelectedProfile.ClientLogo = null;
+            ClientLogoPreview = null;
             await _profileService.UpdateProfileAsync(SelectedProfile);
             ValidationMessage = string.Empty;
         }
@@ -229,6 +242,7 @@ public partial class ProfileManagementViewModel : ObservableObject
 
             var logo = await _brandingService.ImportLogoFromBytesAsync(bytes);
             SelectedProfile.ClientLogo = logo;
+            ClientLogoPreview = FormatLogoPreview(logo);
             await _profileService.UpdateProfileAsync(SelectedProfile);
             ValidationMessage = "Client logo pulled from Entra branding.";
         }