fix(converter): accept a required quantity of 0 when loading drawings

Nest drawings can require no parts, but the CAD Converter's quantity
spinner had a minimum of 1, so Edit Drawings in Converter threw
ArgumentOutOfRangeException as soon as such a drawing was selected.
Lowering the minimum to 0 lets the converter load it and hand the
drawing back with its quantity still 0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.WinForms.Tests/Forms/CadConverterQuantityTests.cs

@@ -0,0 +1,45 @@
+using System.Drawing;
+using System.Windows.Forms;
+using OpenNest.Bending;
+using OpenNest.Forms;
+using OpenNest.Geometry;
+using OpenNest.IO;
+
+namespace OpenNest.WinForms.Tests.Forms;
+
+/// <summary>
+/// A nest drawing may require no parts (quantity 0); editing it in the CAD Converter must keep that.
+/// </summary>
+public class CadConverterQuantityTests
+{
+    [Fact]
+    public void LoadingADrawingThatRequiresNoParts_KeepsItsQuantityAtZero() => ArchUnitsTests.RunSta(() =>
+    {
+        using var form = new CadConverterForm();
+        form.WindowState = FormWindowState.Normal;
+        form.StartPosition = FormStartPosition.Manual;
+        form.Location = new Point(0, 0);
+        form.Size = new System.Drawing.Size(1200, 800);
+        form.Show();
+
+        form.LoadDrawings([Square(0)]);
+
+        var drawing = Assert.Single(form.GetDrawings());
+        Assert.Equal(0, drawing.Quantity.Required);
+        form.Close();
+    });
+
+    private static Drawing Square(int quantity)
+    {
+        var entities = new List<Entity>
+        {
+            new Line(0, 0, 10, 0), new Line(10, 0, 10, 10), new Line(10, 10, 0, 10), new Line(0, 10, 0, 0),
+        };
+        foreach (var e in entities)
+            e.Layer = new Layer("0") { IsVisible = true };
+        var drawing = CadImporter.BuildDrawing(new CadImportResult { Entities = entities, Name = "square" }, entities,
+            new List<Bend>(), 1, null, null);
+        drawing.Quantity.Required = quantity;
+        return drawing;
+    }
+}

OpenNest/Forms/CadConverterForm.Designer.cs

@@ -200,7 +200,7 @@ namespace OpenNest.Forms
             numQuantity.Location = new System.Drawing.Point(37, 6);
             numQuantity.Margin = new System.Windows.Forms.Padding(2, 0, 8, 0);
             numQuantity.Maximum = new decimal(new int[] { 9999, 0, 0, 0 });
-            numQuantity.Minimum = new decimal(new int[] { 1, 0, 0, 0 });
+            numQuantity.Minimum = new decimal(new int[] { 0, 0, 0, 0 });
             numQuantity.Name = "numQuantity";
             numQuantity.Size = new System.Drawing.Size(50, 23);
             numQuantity.TabIndex = 1;