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/market_screen.dart

@@ -493,7 +493,7 @@ class _MarketFilterBar extends StatelessWidget {
           avatar: Icon(
             Icons.eco,
             size: 18,
-            color: state.organicOnly ? null : seedGreen,
+            color: state.organicOnly ? Colors.white : seedGreen,
           ),
           label: Text(t.editVariety.organic),
           selected: state.organicOnly,