feat(lists): assign a colour to a sidebar folder

Tints the folder name and its guide line; picked from a swatch row in the folder settings.

src/ClaudeDo.Ui/Converters/SimpleConverters.cs

@@ -9,6 +9,16 @@ namespace ClaudeDo.Ui.Converters;
 // instantiate a generic FuncValueConverter as a resource; they stay one per type so the
 // x:Key names in App.axaml keep resolving. Anything with real logic gets its own file.
 
+/// <summary>"#RRGGBB" to a brush; anything else falls back to the app resource named by the parameter.</summary>
+public sealed class HexBrushConverter : OneWayConverter
+{
+    public static HexBrushConverter Instance { get; } = new();
+    public override object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value is string s && Color.TryParse(s, out var c)
+            ? new SolidColorBrush(c)
+            : Avalonia.Application.Current?.TryGetResource(parameter as string ?? "", null, out var res) == true ? res : null;
+}
+
 public sealed class UpperCaseConverter : OneWayConverter
 {
     public static UpperCaseConverter Instance { get; } = new();