feat: apply iOS-style theme and replace removed color constants

Switch AppColors to iOS system palette (007AFF blue, F2F2F7 grouped
background, separator, label hierarchy) and rewrite AppTheme with
iOS-inspired Material 3 tokens (no elevation, negative letter-spacing,
50px buttons, 12px radii). Replace removed primaryLight/accent references
in recipe screens with primary.withValues(alpha:0.15) and primary.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/features/recipes/recipe_detail_screen.dart

@@ -194,7 +194,7 @@ class _RecipeDetailScreenState extends ConsumerState<RecipeDetailScreen> {
 class _PlaceholderImage extends StatelessWidget {
   @override
   Widget build(BuildContext context) => Container(
-        color: AppColors.primaryLight.withValues(alpha: 0.3),
+        color: AppColors.primary.withValues(alpha: 0.15),
         child: const Center(child: Icon(Icons.restaurant, size: 64)),
       );
 }
@@ -293,7 +293,7 @@ class _NutritionCard extends StatelessWidget {
                   child: Text(
                     '≈',
                     style: TextStyle(
-                      color: AppColors.accent,
+                      color: AppColors.primary,
                       fontSize: 16,
                       fontWeight: FontWeight.bold,
                     ),
@@ -360,7 +360,7 @@ class _TagsRow extends StatelessWidget {
           .map(
             (t) => Chip(
               label: Text(t, style: const TextStyle(fontSize: 11)),
-              backgroundColor: AppColors.primaryLight.withValues(alpha: 0.3),
+              backgroundColor: AppColors.primary.withValues(alpha: 0.15),
               padding: EdgeInsets.zero,
               visualDensity: VisualDensity.compact,
             ),
@@ -490,12 +490,12 @@ class _StepTile extends StatelessWidget {
                   Row(
                     children: [
                       const Icon(Icons.timer_outlined,
-                          size: 14, color: AppColors.accent),
+                          size: 14, color: AppColors.primary),
                       const SizedBox(width: 4),
                       Text(
                         _formatTimer(step.timerSeconds!),
                         style: const TextStyle(
-                            color: AppColors.accent, fontSize: 12),
+                            color: AppColors.primary, fontSize: 12),
                       ),
                     ],
                   ),