Shorthand properties like background: var(--color) were silently dropped
when pasting into the CSS editor because the shorthand expander can't
handle unresolved var() references.
Fix: CSS vars from the current element's computed styles are now
resolved before expanding shorthands. Inline and nested fallbacks
(var(--a, var(--b))) are supported. If a var still can't be resolved, a
toast error is shown instead of silently dropping the property.