feat: add BomImportForm designer layout and shell

Task 7: WinForms dialog for BOM import with Input groupbox (job name,
BOM file, DXF folder, plate size, Analyze button), Material Groups
DataGridView, and bottom panel (summary label, Create Nests, Close).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest/Forms/BomImportForm.cs

@@ -0,0 +1,47 @@
+using OpenNest.CNC;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+using OpenNest.IO;
+using OpenNest.IO.Bom;
+using System;
+using System.Collections.Generic;
+using System.Data;
+using System.IO;
+using System.Linq;
+using System.Windows.Forms;
+
+namespace OpenNest.Forms
+{
+    public partial class BomImportForm : Form
+    {
+        private BomAnalysis _analysis;
+
+        public Form MdiParentForm { get; set; }
+
+        public BomImportForm()
+        {
+            InitializeComponent();
+        }
+
+        private void BrowseBom_Click(object sender, EventArgs e)
+        {
+        }
+
+        private void BrowseDxf_Click(object sender, EventArgs e)
+        {
+        }
+
+        private void Analyze_Click(object sender, EventArgs e)
+        {
+        }
+
+        private void CreateNests_Click(object sender, EventArgs e)
+        {
+        }
+
+        private void BtnClose_Click(object sender, EventArgs e)
+        {
+            Close();
+        }
+    }
+}