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));
+    }
+}