feat: add CollapsiblePanel reusable control

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

OpenNest/Controls/CollapsiblePanel.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Drawing;
+using System.Windows.Forms;
+
+namespace OpenNest.Controls
+{
+    public class CollapsiblePanel : Panel
+    {
+        private readonly Panel headerPanel;
+        private readonly Label headerLabel;
+        private readonly Label chevronLabel;
+        private readonly Panel contentPanel;
+        private bool isExpanded;
+        private int expandedHeight;
+
+        public CollapsiblePanel()
+        {
+            isExpanded = true;
+            expandedHeight = 200;
+
+            headerPanel = new Panel
+            {
+                Dock = DockStyle.Top,
+                Height = 28,
+                BackColor = Color.FromArgb(240, 240, 240),
+                Cursor = Cursors.Hand
+            };
+
+            chevronLabel = new Label
+            {
+                Text = "▾",
+                AutoSize = false,
+                Size = new Size(20, 28),
+                Dock = DockStyle.Left,
+                TextAlign = ContentAlignment.MiddleCenter,
+                Font = new Font("Segoe UI", 9f)
+            };
+
+            headerLabel = new Label
+            {
+                Text = "Section",
+                AutoSize = false,
+                Dock = DockStyle.Fill,
+                TextAlign = ContentAlignment.MiddleLeft,
+                Font = new Font("Segoe UI", 9f, FontStyle.Bold)
+            };
+
+            headerPanel.Controls.Add(headerLabel);
+            headerPanel.Controls.Add(chevronLabel);
+            headerPanel.Click += (s, e) => Toggle();
+            headerLabel.Click += (s, e) => Toggle();
+            chevronLabel.Click += (s, e) => Toggle();
+
+            contentPanel = new Panel
+            {
+                Dock = DockStyle.Fill
+            };
+
+            Controls.Add(contentPanel);
+            Controls.Add(headerPanel);
+        }
+
+        public string HeaderText
+        {
+            get => headerLabel.Text;
+            set => headerLabel.Text = value;
+        }
+
+        public bool IsExpanded
+        {
+            get => isExpanded;
+            set
+            {
+                isExpanded = value;
+                UpdateLayout();
+            }
+        }
+
+        public int ExpandedHeight
+        {
+            get => expandedHeight;
+            set
+            {
+                expandedHeight = value;
+                if (isExpanded) Height = value;
+            }
+        }
+
+        public Panel ContentPanel => contentPanel;
+
+        public void Toggle()
+        {
+            isExpanded = !isExpanded;
+            UpdateLayout();
+        }
+
+        private void UpdateLayout()
+        {
+            contentPanel.Visible = isExpanded;
+            chevronLabel.Text = isExpanded ? "▾" : "▸";
+            Height = isExpanded ? expandedHeight : headerPanel.Height;
+        }
+    }
+}