Page: Fix page header disappearing under the global header

With Qt 6.7 Controls' page header position is enforced and we can no longer move it below the global header. So instead, we need to trick the header control into placing its content below the global header by overriding the top padding. This generally works when using ToolBar as header type, but isn't as reliable as the original approach. Longer term we should either fix QTBUG-120197 upstream or not make the global toolbar part of the page.