UI/Qt: Restore horizontal tab widths after turning off vertical tabs

After commit a7e68ef, When switching from vertical tabs to horizontal
tabs, the QTabBar could keep its stale narrow vertical width. This made
horizontal tabs render too small and, with multiple tabs, caused scroll
buttons to appear even though there was enough room in the tab strip.

Make the horizontal tab bar width explicit from the same available-width
calculation used for tab size hints.