fix(formmain): 修复弹窗时按下回车导致崩溃的问题 (#2805)

（由 GPT-5.4-mini 生成）

Author: Big-Cake-jpg

Plain Craft Launcher 2/FormMain.xaml.cs

@@ -748,7 +748,17 @@ private void FormMain_KeyDown(object sender, KeyEventArgs e)
         {
             if (e.Key == Key.Enter)
             {
-                ((MyMsgInput)PanMsg.Children[0]).Btn1_Click(sender, null);
+                var msg = PanMsg.Children[0];
+                Action? enterAction = msg switch
+                {
+                    MyMsgInput input => () => input.Btn1_Click(sender, null),
+                    MyMsgSelect select => () => select.Btn1_Click(sender, null),
+                    MyMsgText text => () => text.Btn1_Click(sender, null),
+                    MyMsgMarkdown markdown => () => markdown.Btn1_Click(sender, null),
+                    MyMsgLogin login => () => login.Btn1_Click(sender, null),
+                    _ => null
+                };
+                enterAction?.Invoke();
                 return;
             }