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.

internal/ui/static/css/common.css

@@ -1106,6 +1106,12 @@ article.category-has-unread {
     margin-bottom: 10px;
 }
 
+.entry-content h1,
+.entry-content h2,
+.entry-content h3 {
+    line-height: 1.2;
+}
+
 .entry-content iframe,
 .entry-content video,
 .entry-content img {