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,
               ),
             );

apps/app_seeds/lib/ui/inventory_list_screen.dart

@@ -308,7 +308,7 @@ class _FilterBar extends StatelessWidget {
           avatar: Icon(
             Icons.calendar_month,
             size: 18,
-            color: state.sowThisMonthOnly ? null : seedGreen,
+            color: state.sowThisMonthOnly ? Colors.white : seedGreen,
           ),
           label: Text(t.calendar.filterChip),
           selected: state.sowThisMonthOnly,
@@ -320,7 +320,7 @@ class _FilterBar extends StatelessWidget {
           avatar: Icon(
             Icons.volunteer_activism_outlined,
             size: 18,
-            color: state.sharingOnly ? null : seedGreen,
+            color: state.sharingOnly ? Colors.white : seedGreen,
           ),
           label: Text(t.share.filterChip),
           selected: state.sharingOnly,
@@ -332,7 +332,7 @@ class _FilterBar 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,
@@ -344,7 +344,7 @@ class _FilterBar extends StatelessWidget {
           avatar: Icon(
             Icons.autorenew,
             size: 18,
-            color: state.needsReproductionOnly ? null : seedGreen,
+            color: state.needsReproductionOnly ? Colors.white : seedGreen,
           ),
           label: Text(t.inventory.needsReproductionFilter),
           selected: state.needsReproductionOnly,
@@ -446,16 +446,23 @@ class _SwatchFilterChip extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    // Idle: a soft same-hue fill with dark ink. Selected: the ink becomes the
+    // fill and the content turns white, so the chip clearly reads as "on" while
+    // keeping its family/form hue. White on every ink meets WCAG AA — see
+    // test/ui/category_palette_contrast_test.dart.
+    final ink = selected ? Colors.white : swatch.ink;
     return FilterChip(
       label: Text(label),
-      avatar: icon == null ? null : Icon(icon, size: 18, color: swatch.ink),
+      avatar: icon == null ? null : Icon(icon, size: 18, color: ink),
       selected: selected,
       onSelected: (_) => onSelected(),
       backgroundColor: swatch.fill,
-      selectedColor: swatch.ink.withValues(alpha: 0.24),
-      checkmarkColor: swatch.ink,
-      side: BorderSide(color: swatch.ink.withValues(alpha: 0.35)),
-      labelStyle: TextStyle(color: swatch.ink, fontWeight: FontWeight.w500),
+      selectedColor: swatch.ink,
+      checkmarkColor: Colors.white,
+      side: BorderSide(
+        color: swatch.ink.withValues(alpha: selected ? 0 : 0.35),
+      ),
+      labelStyle: TextStyle(color: ink, fontWeight: FontWeight.w500),
     );
   }
 }

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,

apps/app_seeds/lib/ui/theme.dart

@@ -105,5 +105,24 @@ ThemeData buildTaneTheme() {
       thickness: 1,
       color: seedDivider,
     ),
+    // Filter/choice chips: idle chips stay light with dark ink, but a *selected*
+    // chip fills solid green with white ink so "on" reads unmistakably against
+    // the pale canvas (the soft tonal default sat too close to it). White on
+    // seedGreen meets WCAG AA — see test/ui/theme_contrast_test.dart. Chips that
+    // carry their own hue (family/form swatches) override this per-swatch.
+    chipTheme: const ChipThemeData(
+      selectedColor: seedGreen,
+      checkmarkColor: Colors.white,
+      labelStyle: TextStyle(
+        color: seedOnSurface,
+        fontSize: 14,
+        fontWeight: FontWeight.w500,
+      ),
+      secondaryLabelStyle: TextStyle(
+        color: Colors.white,
+        fontSize: 14,
+        fontWeight: FontWeight.w500,
+      ),
+    ),
   );
 }