msgviewer: cleanup PartSwitcher on :toggle-headers

Cleanup existing PartSwitcher before recreating a new one when command
:toggle-headers is executed, ensuring existing part pagers are cleaned
up.  This fixes a leak in pager processes when :toggle-headers is
executed repeatedly without closing the message.

Signed-off-by: Jose Lombera <jose@lombera.dev>
Acked-by: Koni Marti <koni.marti@gmail.com>

widgets/msgviewer.go

@@ -308,6 +308,7 @@ func (mv *MessageViewer) MarkedMessages() ([]uint32, error) {
 
 func (mv *MessageViewer) ToggleHeaders() {
 	switcher := mv.switcher
+	switcher.Cleanup()
 	mv.conf.Viewer.ShowHeaders = !mv.conf.Viewer.ShowHeaders
 	err := createSwitcher(mv.acct, switcher, mv.conf, mv.msg)
 	if err != nil {