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/Views/Converters/Base64ToImageSourceConverter.cs

@@ -0,0 +1,46 @@
+using System.Globalization;
+using System.IO;
+using System.Windows.Data;
+using System.Windows.Media.Imaging;
+
+namespace SharepointToolbox.Views.Converters;
+
+/// <summary>
+/// Converts a data URI string (e.g. "data:image/png;base64,iVBOR...") to a BitmapImage
+/// for use with WPF Image controls. Returns null for null, empty, or malformed input.
+/// </summary>
+[ValueConversion(typeof(string), typeof(BitmapImage))]
+public class Base64ToImageSourceConverter : IValueConverter
+{
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (value is not string dataUri || string.IsNullOrEmpty(dataUri))
+            return null;
+
+        try
+        {
+            var marker = "base64,";
+            var idx = dataUri.IndexOf(marker, StringComparison.Ordinal);
+            if (idx < 0) return null;
+
+            var base64 = dataUri[(idx + marker.Length)..];
+            var bytes = System.Convert.FromBase64String(base64);
+
+            var image = new BitmapImage();
+            using var ms = new MemoryStream(bytes);
+            image.BeginInit();
+            image.CacheOption = BitmapCacheOption.OnLoad;
+            image.StreamSource = ms;
+            image.EndInit();
+            image.Freeze();
+            return image;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotImplementedException();
+}