fix(css): set line-height on article headings to prevent overlap when they wrap

Article headings inherit .entry-content's fixed line-height: 1.4em (computed at
the body font-size), which is smaller than the heading font-size, so headings
that wrap onto two lines overlap. Add an explicit unitless line-height: 1.2
scoped to .entry-content headings so the leading scales with each heading's own
font-size, without affecting Miniflux's own UI headings.

Fixes #4399.