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/features/settings/ui/screens/features_screen.dart

@@ -18,7 +18,7 @@ class _FeaturesScreenState extends State<FeaturesScreen> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: const Color(0xFFF8FAFC),
+      backgroundColor: AppColors.surfaceSecondary,
       body: SafeArea(
         child: Column(
           children: [
@@ -90,7 +90,7 @@ class _FeaturesScreenState extends State<FeaturesScreen> {
       children: [
         _buildFeatureCard(
           icon: Icons.calendar_view_week,
-          iconColor: const Color(0xFF10B981),
+          iconColor: AppColors.success,
           iconBg: const Color(0xFFECFDF5),
           iconBorder: const Color(0xFFCDEEDC),
           title: '周报生成',
@@ -101,7 +101,7 @@ class _FeaturesScreenState extends State<FeaturesScreen> {
         const SizedBox(height: 10),
         _buildFeatureCard(
           icon: Icons.article,
-          iconColor: const Color(0xFFF59E0B),
+          iconColor: AppColors.warning,
           iconBg: const Color(0xFFFFF7ED),
           iconBorder: const Color(0xFFFDE6CD),
           title: '每周摘要',