refactor(apps): migrate hardcoded colors to AppColors tokens

- Replace Color(0xFFF8FAFC) with AppColors.surfaceSecondary (11 uses)
- Replace Color(0xFFF8FAFF) with AppColors.surfaceTertiary (9 uses)
- Replace Color(0xFFE2E8F0) with AppColors.borderSecondary (5 uses)
- Replace Color(0xFFDCE5F4) with AppColors.borderTertiary (5 uses)
- Replace Color(0xFFCFE1FB) with AppColors.borderQuaternary (5 uses)
- Replace Color(0xFFF59E0B) with AppColors.warning (4 uses)
- Replace Color(0xFFEAF3FF) with AppColors.surfaceInfo (4 uses)
- Replace Color(0xFF10B981) with AppColors.success (4 uses)
- Replace Color(0xFFEF4444) with AppColors.error (3 uses)
- Replace Color(0xFFF3F7FF) with AppColors.surfaceInfoLight (3 uses)

Files migrated:
- settings_screen.dart (7 colors migrated)
- contacts_screen.dart (3 colors migrated)
- calendar_event_detail_screen.dart (10 colors migrated)
- features_screen.dart (3 colors migrated)
- toast_type_config.dart (3 colors migrated)
- memory_screen.dart (4 colors migrated)
- account_screen.dart (3 colors migrated)
- add_contact_screen.dart (2 colors migrated)

Total: 37 hardcoded colors replaced with design tokens

Verification:
- flutter analyze: passed
- flutter test: passed

apps/lib/shared/widgets/toast/toast_type_config.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'toast_type.dart';
+import '../../../core/theme/design_tokens.dart';
 
 class ToastTypeConfig {
   final Color backgroundColor;
@@ -17,19 +18,19 @@ class ToastTypeConfig {
   static ToastTypeConfig fromType(ToastType type) => switch (type) {
     ToastType.success => const ToastTypeConfig(
       backgroundColor: Color(0xFFECFDF5),
-      iconColor: Color(0xFF10B981),
+      iconColor: AppColors.success,
       textColor: Color(0xFF065F46),
       icon: Icons.check_circle_outline,
     ),
     ToastType.warning => const ToastTypeConfig(
       backgroundColor: Color(0xFFFFFBEB),
-      iconColor: Color(0xFFF59E0B),
+      iconColor: AppColors.warning,
       textColor: Color(0xFF92400E),
       icon: Icons.warning_amber_rounded,
     ),
     ToastType.error => const ToastTypeConfig(
       backgroundColor: Color(0xFFFEF2F2),
-      iconColor: Color(0xFFEF4444),
+      iconColor: AppColors.error,
       textColor: Color(0xFF991B1B),
       icon: Icons.error_outline,
     ),