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.Tests/Converters/Base64ToImageSourceConverterTests.cs

@@ -0,0 +1,53 @@
+using System.Globalization;
+using SharepointToolbox.Views.Converters;
+
+namespace SharepointToolbox.Tests.Converters;
+
+[Trait("Category", "Unit")]
+public class Base64ToImageSourceConverterTests
+{
+    private readonly Base64ToImageSourceConverter _converter = new();
+
+    [Fact]
+    public void Convert_NullValue_ReturnsNull()
+    {
+        var result = _converter.Convert(null, typeof(object), null, CultureInfo.InvariantCulture);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void Convert_EmptyString_ReturnsNull()
+    {
+        var result = _converter.Convert(string.Empty, typeof(object), null, CultureInfo.InvariantCulture);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void Convert_NonStringValue_ReturnsNull()
+    {
+        var result = _converter.Convert(42, typeof(object), null, CultureInfo.InvariantCulture);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void Convert_MalformedString_NoBase64Marker_ReturnsNull()
+    {
+        var result = _converter.Convert("not-a-data-uri", typeof(object), null, CultureInfo.InvariantCulture);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void Convert_InvalidBase64AfterMarker_ReturnsNull()
+    {
+        // Has the marker but invalid base64 content — should not throw
+        var result = _converter.Convert("data:image/png;base64,!!!invalid!!!", typeof(object), null, CultureInfo.InvariantCulture);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void ConvertBack_ThrowsNotImplementedException()
+    {
+        Assert.Throws<NotImplementedException>(() =>
+            _converter.ConvertBack(null, typeof(object), null, CultureInfo.InvariantCulture));
+    }
+}

SharepointToolbox.Tests/ViewModels/ProfileManagementViewModelLogoTests.cs

@@ -115,4 +115,71 @@ public class ProfileManagementViewModelLogoTests : IDisposable
         var persisted = profiles.First(p => p.Name == "TestTenant");
         Assert.Null(persisted.ClientLogo);
     }
+
+    [Fact]
+    public void ClientLogoPreview_IsNull_WhenNoProfileSelected()
+    {
+        var vm = CreateViewModel();
+        Assert.Null(vm.ClientLogoPreview);
+    }
+
+    [Fact]
+    public void ClientLogoPreview_UpdatesToDataUri_WhenProfileWithLogoSelected()
+    {
+        var vm = CreateViewModel();
+        var profile = new TenantProfile
+        {
+            Name = "WithLogo",
+            TenantUrl = "https://test.sharepoint.com",
+            ClientId = "00000000-0000-0000-0000-000000000002",
+            ClientLogo = new LogoData { Base64 = "dGVzdA==", MimeType = "image/png" }
+        };
+
+        vm.SelectedProfile = profile;
+
+        Assert.Equal("data:image/png;base64,dGVzdA==", vm.ClientLogoPreview);
+    }
+
+    [Fact]
+    public void ClientLogoPreview_IsNull_WhenProfileWithoutLogoSelected()
+    {
+        var vm = CreateViewModel();
+        var profile = new TenantProfile
+        {
+            Name = "NoLogo",
+            TenantUrl = "https://test.sharepoint.com",
+            ClientId = "00000000-0000-0000-0000-000000000003"
+        };
+
+        vm.SelectedProfile = profile;
+
+        Assert.Null(vm.ClientLogoPreview);
+    }
+
+    [Fact]
+    public async Task ClearClientLogoCommand_SetsClientLogoPreviewToNull()
+    {
+        var profileService = new ProfileService(new ProfileRepository(_tempFile));
+        var profile = new TenantProfile
+        {
+            Name = "ClearTest",
+            TenantUrl = "https://test.sharepoint.com",
+            ClientId = "00000000-0000-0000-0000-000000000004",
+            ClientLogo = new LogoData { Base64 = "dGVzdA==", MimeType = "image/png" }
+        };
+        await profileService.AddProfileAsync(profile);
+
+        var vm = new ProfileManagementViewModel(
+            profileService,
+            _mockBranding.Object,
+            _graphClientFactory,
+            _logger);
+
+        vm.SelectedProfile = profile;
+        Assert.NotNull(vm.ClientLogoPreview);
+
+        await vm.ClearClientLogoCommand.ExecuteAsync(null);
+
+        Assert.Null(vm.ClientLogoPreview);
+    }
 }