refactor(theme): tighten the colour helper API

Address review feedback on the new colour system.

Drop getAvailabilityColor's separate brightness parameter and read it from
the ColorScheme instead, so a caller can no longer pass a dark scheme
alongside Brightness.light. Nothing outside this PR calls it yet, so the
signature is free to change now.

Correct getAccentColor's doc comment: the unknown-category fallback is
adapted for dark surfaces like any other hue, so the dark fallback is a
lifted navy rather than the navy literal the comment claimed.

No behaviour change; goldens unaffected.

Author: claude

lib/screens/my_festival_screen.dart

@@ -504,7 +504,6 @@ class _MyFestivalScreenState extends State<MyFestivalScreen> {
     final color = CategoryColorHelper.getAvailabilityColor(
       atRisk,
       theme.colorScheme,
-      theme.brightness,
     );
     return Container(
       padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 4),

lib/utils/category_color_helper.dart

@@ -70,8 +70,11 @@ class CategoryColorHelper {
   /// rows and the similar-drinks carousel.
   ///
   /// Derived from [brightness]: the fixed hue in light mode, a lightness-lifted
-  /// variant in dark mode so the edge reads against a dark surface. Falls back
-  /// to CBF navy for unknown categories.
+  /// variant in dark mode so the edge reads against a dark surface.
+  ///
+  /// An unrecognised category falls back to CBF navy, which is adapted for
+  /// dark surfaces the same way a real category is — so the dark fallback is a
+  /// lifted navy, not the navy literal.
   static Color getAccentColor(String category, Brightness brightness) {
     final hue = _categoryHues[category] ?? _fallbackHue;
     return brightness == Brightness.dark ? _liftForDark(hue) : hue;
@@ -109,12 +112,14 @@ class CategoryColorHelper {
   /// should track the app's error language, so [colorScheme] is required.
   /// Every other state uses a fixed light/dark pair chosen for legibility on
   /// both surfaces.
+  ///
+  /// Brightness is read from [colorScheme] rather than taken separately, so a
+  /// caller cannot pass a dark scheme alongside a light brightness.
   static Color getAvailabilityColor(
     AvailabilityStatus status,
     ColorScheme colorScheme,
-    Brightness brightness,
   ) {
-    final isDark = brightness == Brightness.dark;
+    final isDark = colorScheme.brightness == Brightness.dark;
     switch (status) {
       case AvailabilityStatus.plenty:
         return isDark ? const Color(0xFF4CAF50) : const Color(0xFF2E7D32);

lib/widgets/drink_card.dart

@@ -273,7 +273,6 @@ class _AvailabilityChip extends StatelessWidget {
     final color = CategoryColorHelper.getAvailabilityColor(
       status,
       theme.colorScheme,
-      theme.brightness,
     );
     String label;
     IconData icon;

test/category_color_helper_test.dart

@@ -149,20 +149,12 @@ void main() {
       };
       for (final entry in expected.entries) {
         expect(
-          CategoryColorHelper.getAvailabilityColor(
-            entry.key,
-            lightScheme,
-            Brightness.light,
-          ),
+          CategoryColorHelper.getAvailabilityColor(entry.key, lightScheme),
           entry.value.$1,
           reason: '${entry.key} light',
         );
         expect(
-          CategoryColorHelper.getAvailabilityColor(
-            entry.key,
-            darkScheme,
-            Brightness.dark,
-          ),
+          CategoryColorHelper.getAvailabilityColor(entry.key, darkScheme),
           entry.value.$2,
           reason: '${entry.key} dark',
         );
@@ -174,15 +166,13 @@ void main() {
         CategoryColorHelper.getAvailabilityColor(
           AvailabilityStatus.out,
           lightScheme,
-          Brightness.light,
         ),
         lightScheme.error,
       );
       expect(
         CategoryColorHelper.getAvailabilityColor(
           AvailabilityStatus.out,
           darkScheme,
-          Brightness.dark,
         ),
         darkScheme.error,
       );
@@ -191,11 +181,7 @@ void main() {
     test('covers every AvailabilityStatus', () {
       for (final status in AvailabilityStatus.values) {
         expect(
-          () => CategoryColorHelper.getAvailabilityColor(
-            status,
-            lightScheme,
-            Brightness.light,
-          ),
+          () => CategoryColorHelper.getAvailabilityColor(status, lightScheme),
           returnsNormally,
         );
       }