polish(ux): selected filter chips fill solid for clear contrast

The selected state used a pale tonal container (seedPrimaryContainer)
that sat almost on top of the pale canvas, so 'on' barely read. Selected
chips now fill solid — seedGreen for the plain attribute/market chips and
the swatch's own ink for family/form chips — with white label, icon and
checkmark. Idle chips stay light with dark ink, so the on/off jump is
unmistakable. White-on-ink AA is locked by a new contrast test.

apps/app_seeds/lib/ui/calendar_screen.dart

@@ -164,9 +164,9 @@ class _MonthStripState extends State<_MonthStrip> {
               ),
               selected: selected,
               onSelected: (_) => widget.onSelect(m),
-              selectedColor: seedPrimaryContainer,
+              selectedColor: seedGreen,
               labelStyle: TextStyle(
-                color: selected ? seedOnPrimaryContainer : seedMuted,
+                color: selected ? Colors.white : seedMuted,
                 fontWeight: selected ? FontWeight.w600 : FontWeight.w400,
               ),
             );