feat(shapes): generate unique drawing names from parameters and add toolbar button

Shape library drawings now get descriptive names based on their
parameters (e.g. "Rectangle 12x6", "Circle 8 Dia") instead of generic
type names, preventing silent duplicates in the DrawingCollection
HashSet. Added a Shape Library button to the Drawings tab toolbar
and removed separators between toolbar buttons for a cleaner look.

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

OpenNest.Core/Shapes/TShape.cs

@@ -10,6 +10,8 @@ namespace OpenNest.Shapes
         public double StemWidth { get; set; }
         public double BarHeight { get; set; }
 
+        public override string GenerateName() => $"T {Dim(Width)}x{Dim(Height)}";
+
         public override void SetPreviewDefaults()
         {
             Width = 10;