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/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,
+      ),
+    ),
   );
 }