feat: 添加白板模式自动全屏功能

- 新增"白板模式自动全屏"设置选项
- 进入白板模式时自动全屏，退出时恢复工作区域大小
- 需配合"避免全屏助手"功能使用
- 用户可独立控制白板模式的全屏行为

Ink Canvas/MainWindow_cs/MW_FloatingBarIcons.cs

@@ -3015,6 +3015,22 @@ namespace Ink_Canvas
                         ClearStrokes(true);
                         RestoreStrokes(true);
 
+                        // 退出白板模式时取消全屏
+                        if (Settings.Advanced.IsEnableAvoidFullScreenHelper && Settings.Advanced.IsEnableAvoidFullScreenHelperForWhiteboard)
+                        {
+                            // 恢复为非画板模式，重新启用全屏限制
+                            AvoidFullScreenHelper.SetBoardMode(false);
+
+                            Dispatcher.BeginInvoke(new Action(() =>
+                            {
+                                // 退出白板模式，恢复到工作区域大小
+                                var workingArea = System.Windows.Forms.Screen.PrimaryScreen.WorkingArea;
+                                MainWindow.MoveWindow(new WindowInteropHelper(this).Handle, 
+                                    workingArea.Left, workingArea.Top,
+                                    workingArea.Width, workingArea.Height, true);
+                            }), DispatcherPriority.ApplicationIdle);
+                        }
+
                         // 在屏幕模式下恢复基础浮动栏的显示
                         ViewboxFloatingBar.Visibility = Visibility.Visible;
 
@@ -3068,6 +3084,19 @@ namespace Ink_Canvas
 
                         RestoreStrokes();
 
+                        // 进入白板模式时全屏
+                        if (Settings.Advanced.IsEnableAvoidFullScreenHelper && Settings.Advanced.IsEnableAvoidFullScreenHelperForWhiteboard)
+                        {
+                            // 设置为画板模式，允许全屏操作
+                            AvoidFullScreenHelper.SetBoardMode(true);
+                            Dispatcher.BeginInvoke(new Action(() =>
+                            {
+                                MainWindow.MoveWindow(new WindowInteropHelper(this).Handle, 0, 0,
+                                    System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width,
+                                    System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height, true);
+                            }), DispatcherPriority.ApplicationIdle);
+                        }
+
                         ViewboxFloatingBar.Visibility = Visibility.Collapsed;
 
                         BtnSwitch.Content = "屏幕";