Pin preferences dialog size.

While unfortunate (we generally frown on having fixed sizes in
favour of having everything auto-layout), in this case it's a
cost of lazy-loading the panels.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/14785

(cherry picked from commit cbf83c4161)

common/eda_base_frame.cpp

@@ -1169,6 +1169,10 @@ void EDA_BASE_FRAME::ShowPreferences( wxString aStartPage, wxString aStartParent
     if( !aStartPage.IsEmpty() )
         dlg.SetInitialPage( aStartPage, aStartParentPage );
 
+    // This is unfortunate, but it's the cost of lazy-loading the panels
+    book->SetMinSize( wxSize( 980, 560 ) );
+    book->SetInitialSize( wxSize( 980, 560 ) );
+
     dlg.SetEvtHandlerEnabled( true );
     wxEndBusyCursor();