Set Content property in AdvancedGrid constructor

The Content property is now set to the _grid object, ensuring the grid is properly displayed in the UI. This update solves an issue where the grid was not visible due to the missing Content assignment.

Frank.Wpf.Controls.Grid/AdvancedGrid.cs

@@ -20,6 +20,8 @@ public AdvancedGrid(int columns, int rows)
             _cells[i, j] = new Cell(new CellPosition(i, j));
 
         foreach (var cell in _cells) _grid.Children.Add(cell);
+
+        Content = _grid;
     }
 
     /// <summary>