web/components: stop ButtonGroup doubling its selected segment's edge
a joined group's segments each drew their own 1px top/bottom border in the same
token as the group's own, and the two lines landed adjacent rather than
overlapping, so the selected tab read as a 2px edge with the ::before inset
shadow stacking a third band under it. the -my-px meant to pull them together
never applied: items-center left the segments at their natural height instead of
stretching, so the margins only shrank the flex line the group had already
outgrown.
follows the appview's .btn-group instead — the group is the only thing drawing a
horizontal border, its segments take border-y-0 and stretch to fill the inner
box. the x seams are unchanged, still one divider each via -ml-px.
the recessed surface also needed dark:bg-background-canvas: subtle sits below
default in light (#f9fafb vs #fff) but above it in dark (#374151 vs #1f2937), so
an unselected segment came out lighter than the selected one. canvas (#111827)
is the appview's gray-900.
Signed-off-by: eti <eti@eti.tf>