feat: add Shape Library UI with configurable shapes and flange presets

Add a Shape Library dialog (Nest > Shape Library) for creating drawings
from built-in parametric shapes. Supports configuration presets loaded
from JSON files — ships with 136 standard pipe flanges. Parameters use
TextBox inputs with architectural unit parsing (feet/inches, fractions).

- ShapeLibraryForm with split layout: shape list, preview, parameters
- ShapePreviewControl for auto-zoom rendering with info overlay
- ArchUnits utility for parsing architectural measurements
- SetPreviewDefaults() on all ShapeDefinition subclasses
- Convention-based config discovery (Configurations/{ShapeName}.json)

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

OpenNest.Core/Shapes/RectangleShape.cs

@@ -8,6 +8,12 @@ namespace OpenNest.Shapes
         public double Length { get; set; }
         public double Width { get; set; }
 
+        public override void SetPreviewDefaults()
+        {
+            Length = 12;
+            Width = 6;
+        }
+
         public override Drawing GetDrawing()
         {
             var entities = new List<Entity>