Fix SizedFrame.set_content_padding() to use correct SetSizerProps API

Previous commit incorrectly changed from SetSizerProps to SetSizerProp.
The wxPython API requires border value to be a tuple (directions, amount),
not just an int. Reverted to correct API: SetSizerProps(border=("all", padding))

Fixes TypeError: cannot unpack non-iterable int object

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ctoth

gui_builder/widgets/wx_widgets.py

@@ -1623,7 +1623,7 @@ def set_content_padding(self, padding: int) -> None:
             padding (int): Padding in pixels around all content
         """
         pane: sc.SizedPanel = self.control.GetContentsPane()
-        pane.SetSizerProp("border", padding)
+        pane.SetSizerProps(border=("all", padding))
 
 
 class MDIParentFrame(BaseFrame[FieldType, wx.MDIParentFrame]):