Restructure project layout to flatten directory structure

Move all projects from Source/ to repository root for simpler navigation.
- Remove External/ dependency DLLs (will use NuGet packages)
- Remove Installer/ NSIS script
- Replace PartCollection/PlateCollection with ObservableList
- Add packages.config for NuGet dependencies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

OpenNest/MdiExtensions.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Windows.Forms;
+
+namespace OpenNest
+{
+    public static class MdiExtensions
+    {
+        /// <summary>
+        /// Sets the display state of the 3D bevel on MDI client area.
+        /// Source: http://stackoverflow.com/questions/7752696/how-to-remove-3d-border-sunken-from-mdiclient-component-in-mdi-parent-form
+        /// </summary>
+        /// <param name="form"></param>
+        /// <param name="show"></param>
+        /// <returns></returns>
+        public static bool SetBevel(this Form form, bool show)
+        {
+            foreach (Control c in form.Controls)
+            {
+                if (c is MdiClient == false)
+                    continue;
+
+                var client = c as MdiClient;
+
+                int windowLong = Win32.GetWindowLong(c.Handle, Win32.GWL_EXSTYLE);
+
+                if (show)
+                    windowLong |= Win32.WS_EX_CLIENTEDGE;
+                else
+                    windowLong &= ~Win32.WS_EX_CLIENTEDGE;
+
+                Win32.SetWindowLong(c.Handle, Win32.GWL_EXSTYLE, windowLong);
+
+                // Update the non-client area.
+                Win32.SetWindowPos(client.Handle, IntPtr.Zero, 0, 0, 0, 0,
+                    Win32.SWP_NOACTIVATE | Win32.SWP_NOMOVE | Win32.SWP_NOSIZE | Win32.SWP_NOZORDER |
+                    Win32.SWP_NOOWNERZORDER | Win32.SWP_FRAMECHANGED);
+
+                return true;
+            }
+            return false;
+        }
+    }
+}