fix: 全选后点击添加到白板显示已取消 (#379)

* 调整截图按钮默认行为为选区/白板全屏

* 截图选择器新增添加到白板按钮并支持新建白板页插入

* 修复添加到白板不生效：改为剪贴板复制粘贴插入

* 修复添加到白板截图时序：先截到内存再进白板

* Fix screenshot insert flow honoring add-to-whiteboard option

* Refine screenshot selector freehand confirm and blank double-click full select

* Fix freehand whiteboard insert alpha transparency

* Fix screenshot insert notification text for whiteboard flow

* Adjust screenshot selection: right-click/double-click full-select and keep selector on single click

* Show unmasked freehand selection area like rectangle mode

* Remove freehand cutout mask rendering from screenshot selector

* Restore freehand selected-area unmask behavior in selector

* fix: always restore window visibility after area screenshot

* fix: allow add-to-whiteboard after full-screen selection

Ink Canvas/Windows/ScreenshotSelectorWindow.xaml.cs

@@ -1112,8 +1112,19 @@ namespace Ink_Canvas
                 int screenHeight = (int)(_currentSelection.Height * dpiScale);
 
                 SelectedArea = new DrawingRectangle(screenX, screenY, screenWidth, screenHeight);
+            }
+
+            if (SelectedArea.HasValue)
+            {
                 DialogResult = true;
             }
+            else
+            {
+                HintText.Text = "请先选择截图区域";
+                HintTextBorder.Visibility = Visibility.Visible;
+                return;
+            }
+
             Close();
         }