feat: 浮动栏截图修改为选区截图，添加“添加到白板”按钮 (#377)

* 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

Ink Canvas/MainWindow_cs/MW_FloatingBarIcons.cs

@@ -1130,7 +1130,16 @@ namespace Ink_Canvas
         {
             HideSubPanelsImmediately();
             await Task.Delay(50);
-            SaveScreenShotToDesktop();
+
+            // 白板模式下默认全屏截图到桌面；其余模式默认调用可选区截图
+            if (currentMode == 1)
+            {
+                SaveScreenShotToDesktop();
+            }
+            else
+            {
+                await SaveAreaScreenShotToDesktop();
+            }
         }
 
         /// <summary>
@@ -4309,4 +4318,4 @@ namespace Ink_Canvas
         }
 
     }
-}
+}