mango scroller: make a lone window fill the screen

In mango (mangowc) the single-window scroller case is skipped unless scroller_ignore_proportion_single=0; only then does scroller_default_proportion_single (e.g. 1.0) apply, giving a full-width lone window that drops back to scroller_default_proportion when a second window opens (src/layout/scroll.c). The default is ignore=1, which is why lone windows sit at the normal proportion. Also: validate a generated config with mango -c file -p; XF86AudioPlay and XF86AudioPause binds resolve to the same keycode on a us layout and are flagged as a conflict. env= lines split only at the first comma, so values like GDK_BACKEND=wayland,x11,* are safe.