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/theme.dart

@@ -105,24 +105,5 @@ 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,
-      ),
-    ),
   );
 }