Fix panel tab header visibility issue when single panel is docked

Changed DockPanel DocumentStyle from DockingSdi to DockingWindow to ensure
panel tab headers remain visible even when only one panel exists in a dock area.

This resolves the issue where users could not access panel controls (close button,
pin/auto-hide, dropdown menu) after moving panels to new locations, as the tab
strip was completely hidden with DockingSdi when only one panel was present.

With DockingWindow style:
- Panel name/title always visible
- Close button (X) always accessible
- Pin/auto-hide button always available
- Dropdown menu always accessible
- No more "lost" panels requiring layout reset

Fixes #2960

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

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

mRemoteNG/UI/Forms/frmMain.Designer.cs

@@ -60,7 +60,7 @@ namespace mRemoteNG.UI.Forms
             // 
             this.pnlDock.Dock = System.Windows.Forms.DockStyle.Fill;
             this.pnlDock.DockBackColor = System.Drawing.SystemColors.Control;
-            this.pnlDock.DocumentStyle = WeifenLuo.WinFormsUI.Docking.DocumentStyle.DockingSdi;
+            this.pnlDock.DocumentStyle = WeifenLuo.WinFormsUI.Docking.DocumentStyle.DockingWindow;
             this.pnlDock.Location = new System.Drawing.Point(0, 0);
             this.pnlDock.Name = "pnlDock";
             this.pnlDock.Size = new System.Drawing.Size(1129, 546);