fix(ui): address code review issues in color scheme feature

- Sync PlateView.BackColor on repaint so live scheme switch updates background
- Guard FromHex against truncated hex strings (< 6 chars)
- Cache disk schemes to avoid re-reading Schemes/ folder on every access

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/ColorSchemeRegistry.cs

@@ -17,8 +17,18 @@ namespace OpenNest
                 ["Dark"] = BuildDark()
             };
 
-        public static IEnumerable<ColorScheme> AllSchemes =>
+        private static List<ColorScheme> diskCache;
-            builtIns.Values.Concat(LoadDiskSchemes());
+
+        public static IEnumerable<ColorScheme> AllSchemes
+        {
+            get
+            {
+                diskCache ??= LoadDiskSchemes().ToList();
+                return builtIns.Values.Concat(diskCache);
+            }
+        }
+
+        public static void Refresh() => diskCache = null;
 
         public static ColorScheme Get(string name)
         {

OpenNest/ColorSchemeSerializer.cs

@@ -59,6 +59,8 @@ namespace OpenNest
             if (string.IsNullOrWhiteSpace(hex))
                 return Color.Black;
             var h = hex.TrimStart('#');
+            if (h.Length < 6)
+                return Color.Black;
             var r = byte.Parse(h.Substring(0, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture);
             var g = byte.Parse(h.Substring(2, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture);
             var b = byte.Parse(h.Substring(4, 2), NumberStyles.HexNumber, CultureInfo.InvariantCulture);

OpenNest/Controls/PlateView.cs

@@ -464,6 +464,9 @@ namespace OpenNest.Controls
 
         protected override void OnPaint(PaintEventArgs e)
         {
+            if (BackColor != ColorScheme.BackgroundColor)
+                BackColor = ColorScheme.BackgroundColor;
+
             e.Graphics.SmoothingMode = SmoothingMode.HighSpeed;
 
             if (DrawOrigin)