fix(ui/web): fix editable pill hover visibility in light themes

fix(ui/web): fix editable pill hover visibility in light themes

Refactor pill background to use scoped --_pill-bg and --_pill-bg-hover
custom properties (MD3 pattern). Each variant co-locates its base and
hover background values, and light-mode overrides reassign both.

The hover rule simply consumes var(--_pill-bg-hover), eliminating the
specificity battle that prevented the hover effect from appearing in
light themes.