style(ui): lists icons, section headers, active state

- Add central icon library (Icon.Sun/Activity/Star/Calendar/Eye/Inbox/Folder/Search/Plus/MoreHorizontal/GitBranch/Copy/Trash/Sort/X/Check) to IslandStyles.axaml
- Add list-section-label, search-wrap, kbd, new-list-btn, avatar-circle styles
- Add UpperCaseConverter, IconKeyConverter, DotBrushConverter; register in App.axaml
- Expose SmartLists / UserLists filtered collections from ListsIslandViewModel
- Add DotColorKey (Moss/Peat/Sage rotation) and UserInitials/UserName/MachineName props

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Converters/UpperCaseConverter.cs

@@ -0,0 +1,15 @@
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace ClaudeDo.Ui.Converters;
+
+public class UpperCaseConverter : IValueConverter
+{
+    public static UpperCaseConverter Instance { get; } = new();
+
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value?.ToString()?.ToUpperInvariant();
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}