fix(ui): darken seedMuted and app-bar green to meet WCAG AA contrast

seedMuted (hints, card subtitles, search placeholder) was #8A9880 —
2.62:1 on the canvas and 3.05:1 on white, failing AA for the 13px text
it colours. Darkened to #617057: 5.30:1 on white, 4.56:1 on canvas,
4.88:1 on the field fill, same muted green-grey hue.

The new contrast test also caught the app-bar green: white titles on
#3E8C38 were 4.19:1, so it is nudged to #3A8434 (4.64:1) — a barely
visible change.

test/ui/theme_contrast_test.dart pins the WCAG relative-luminance math
and asserts every text/background pair the palette actually uses.

apps/app_seeds/lib/ui/inventory_list_screen.dart

@@ -233,8 +233,7 @@ class _VarietyTile extends StatelessWidget {
             ),
       trailing: IconButton(
         icon: const Icon(Icons.edit_outlined),
-        // Action colour (≥3:1 on the canvas) so the control stays legible;
-        // seedMuted here fell to 2.62:1.
+        // Action colour: this is a tap target, not secondary text.
         color: seedGreen,
         tooltip: context.t.common.edit,
         onPressed: open,

apps/app_seeds/lib/ui/theme.dart

@@ -6,7 +6,9 @@ import 'package:flutter/material.dart';
 /// headers) and a soft green container for selected/tonal surfaces.
 const seedGreen = Color(0xFF2F7D34); // primary (actions, links, headers)
 const seedGreenDark = Color(0xFF22521E); // status bar / darker green
-const seedAppBar = Color(0xFF3E8C38); // app-bar green
+// App-bar green. Nudged down from the mockups' #3E8C38 so its white title
+// meets WCAG AA (4.5:1) — see test/ui/theme_contrast_test.dart.
+const seedAppBar = Color(0xFF3A8434);
 const seedCanvas = Color(0xFFE7F1E0); // surface / scaffold
 const seedPrimaryContainer = Color(0xFFD3E8C8); // tonal green container
 const seedOnPrimaryContainer = Color(0xFF2F6D2A);
@@ -18,7 +20,10 @@ const seedDivider = Color(0xFFE4E9DD);
 const seedTitle = Color(0xFF1B2416); // heading text on canvas
 const seedOnSurface = Color(0xFF1B2416);
 const seedOnSurfaceVariant = Color(0xFF5C6B52); // scientific names, secondary
-const seedMuted = Color(0xFF8A9880); // hints, list edit icon
+// Hints and secondary text/icons. Darkened from the mockups' #8A9880 (which
+// fell below WCAG AA) to keep ≥4.5:1 on white, the canvas and the field fill —
+// see test/ui/theme_contrast_test.dart.
+const seedMuted = Color(0xFF617057);
 const seedRating = Color(0xFFE8A200); // stars
 
 ThemeData buildTaneTheme() {

apps/app_seeds/test/ui/theme_contrast_test.dart

@@ -0,0 +1,52 @@
+import 'dart:math';
+import 'dart:ui';
+
+import 'package:flutter/material.dart' show Colors;
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/ui/theme.dart';
+
+/// WCAG 2.x relative luminance of a color (sRGB).
+double _luminance(Color c) {
+  double channel(double v) =>
+      v <= 0.03928 ? v / 12.92 : pow((v + 0.055) / 1.055, 2.4).toDouble();
+  return 0.2126 * channel(c.r) + 0.7152 * channel(c.g) + 0.0722 * channel(c.b);
+}
+
+/// WCAG contrast ratio between two colors (order-independent, 1..21).
+double _contrast(Color a, Color b) {
+  final la = _luminance(a);
+  final lb = _luminance(b);
+  final lighter = max(la, lb);
+  final darker = min(la, lb);
+  return (lighter + 0.05) / (darker + 0.05);
+}
+
+void main() {
+  void expectAa(Color foreground, Color background, String label) {
+    expect(
+      _contrast(foreground, background),
+      greaterThanOrEqualTo(4.5),
+      reason: '$label must meet WCAG AA (4.5:1) for normal text',
+    );
+  }
+
+  group('palette meets WCAG AA (4.5:1) for text', () {
+    test('seedMuted (hints, subtitles) on every background it sits on', () {
+      // White: search-field fill and home cards. Canvas/field: M3 surfaces.
+      expectAa(seedMuted, Colors.white, 'seedMuted on white');
+      expectAa(seedMuted, seedCanvas, 'seedMuted on canvas');
+      expectAa(seedMuted, seedField, 'seedMuted on field fill');
+    });
+
+    test('body and secondary text on the canvas', () {
+      expectAa(seedOnSurface, seedCanvas, 'onSurface on canvas');
+      expectAa(seedTitle, seedCanvas, 'titles on canvas');
+      expectAa(seedOnSurfaceVariant, seedCanvas, 'onSurfaceVariant on canvas');
+    });
+
+    test('white text on the green action colours', () {
+      expectAa(Colors.white, seedGreen, 'white on primary green');
+      expectAa(Colors.white, seedAppBar, 'white on app-bar green');
+    });
+  });
+}