fix: correct dock z-order so Fill control gets remaining space

Fill must be at index 0 (front) so it's processed last by the
docking layout engine. Edge docks at higher indices process first.

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

OpenNest/Forms/BomImportForm.Designer.cs

@@ -215,9 +215,9 @@ namespace OpenNest.Forms
             AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
             CancelButton = btnClose;
             ClientSize = new System.Drawing.Size(520, 500);
+            Controls.Add(grpGroups);
             Controls.Add(pnlBottom);
             Controls.Add(grpInput);
-            Controls.Add(grpGroups);
             Font = new System.Drawing.Font("Segoe UI", 9F);
             MinimumSize = new System.Drawing.Size(400, 350);
             MaximizeBox = false;