Dynamic color: keep card surfaces lighter than the page

In light mode surfaceContainerLow is slightly darker than the OS surface bg
maps to, so the selection bar and other cards blended into the page. Use
surfaceContainerLowest (light) / surfaceContainer (dark) for NooColors.surface.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

test/widgets/noo/core_test.dart

@@ -74,6 +74,30 @@ void main() {
     });
   });
 
+  group('NooColors.fromDynamicScheme surfaces', () {
+    for (final brightness in Brightness.values) {
+      test('cards are clearly lighter than the page (${brightness.name})', () {
+        // The OS palette overrides `surface` (what `bg` maps to) but not the
+        // seeded container roles - mimic that mismatch.
+        final seeded = ColorScheme.fromSeed(
+          seedColor: const Color(0xFF3F6B8F),
+          brightness: brightness,
+        );
+        final os = seeded.copyWith(
+          surface: brightness == Brightness.light
+              ? const Color(0xFFF7F9FC)
+              : const Color(0xFF101417),
+        );
+        final c = NooColors.fromDynamicScheme(os);
+        // WCAG-style contrast ratio between the card and the page.
+        final hi = c.surface.computeLuminance();
+        final lo = c.bg.computeLuminance();
+        expect(hi, greaterThan(lo));
+        expect((hi + 0.05) / (lo + 0.05), greaterThan(1.04));
+      });
+    }
+  });
+
   group('NooAvatar', () {
     testNooWidgets('current user uses accent-soft/accent-text', (
       tester,