feat(ui): refactor Settings to TabControl + add Prime Claude tab

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Converters/DateOnlyToDateTimeConverter.cs

@@ -0,0 +1,23 @@
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace ClaudeDo.Ui.Converters;
+
+public sealed class DateOnlyToDateTimeConverter : IValueConverter
+{
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (value is DateOnly d)
+            return d.ToDateTime(TimeOnly.MinValue);
+        return null;
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (value is DateTime dt)
+            return DateOnly.FromDateTime(dt);
+        if (value is DateTimeOffset dto)
+            return DateOnly.FromDateTime(dto.LocalDateTime);
+        return DateOnly.FromDateTime(DateTime.Today);
+    }
+}