polish(ux): drop the checkmark on selected filter chips

The tick overlapped the leading icon on selected chips, reading as a
confusing blob. The solid fill already signals 'on', so hide the
checkmark. Also make the selected label reliably white via an explicit
per-chip style: the global ChipTheme's secondaryLabelStyle was not
applied to FilterChips, leaving dark text on green. Plain attribute/
market chips now share a PlainFilterChip widget; swatch and month chips
just drop their tick.

apps/app_seeds/lib/ui/calendar_screen.dart

@@ -157,6 +157,7 @@ class _MonthStripState extends State<_MonthStrip> {
             final selected = m == widget.month;
             return ChoiceChip(
               key: Key('calendar.month.$m'),
+              showCheckmark: false,
               labelPadding: const EdgeInsets.symmetric(horizontal: 6),
               label: Text(
                 _capitalise(fmt.format(DateTime(2000, m))),