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/contacts/ui/screens/add_contact_screen.dart

@@ -31,7 +31,7 @@ class _AddContactScreenState extends State<AddContactScreen> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: const Color(0xFFF8FAFC),
+      backgroundColor: AppColors.surfaceSecondary,
       body: SafeArea(
         child: Column(
           children: [
@@ -68,9 +68,9 @@ class _AddContactScreenState extends State<AddContactScreen> {
         width: 36,
         height: 36,
         decoration: BoxDecoration(
-          color: const Color(0xFFEAF3FF),
+          color: AppColors.surfaceInfo,
           borderRadius: BorderRadius.circular(18),
-          border: Border.all(color: const Color(0xFFCFE1FB)),
+          border: Border.all(color: AppColors.borderQuaternary),
         ),
         child: const Icon(Icons.check, size: 16, color: AppColors.blue600),
       ),

apps/lib/features/contacts/ui/screens/contacts_screen.dart

@@ -49,7 +49,7 @@ class _ContactsScreenState extends State<ContactsScreen> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: const Color(0xFFF8FAFC),
+      backgroundColor: AppColors.surfaceSecondary,
       body: SafeArea(
         child: Column(
           children: [
@@ -89,7 +89,7 @@ class _ContactsScreenState extends State<ContactsScreen> {
               height: 40,
               padding: const EdgeInsets.symmetric(horizontal: 12),
               decoration: BoxDecoration(
-                color: const Color(0xFFF8FAFF),
+                color: AppColors.surfaceTertiary,
                 borderRadius: BorderRadius.circular(12),
                 border: Border.all(color: const Color(0xFFE4EBF7)),
               ),
@@ -218,7 +218,7 @@ class _ContactsScreenState extends State<ContactsScreen> {
 
   Color _getAvatarBackground(Color color) {
     if (color == AppColors.blue500) return const Color(0xFFEEF4FF);
-    if (color == AppColors.violet600) return const Color(0xFFF3F7FF);
+    if (color == AppColors.violet600) return AppColors.surfaceInfoLight;
     if (color == AppColors.blue600) return const Color(0xFFEDF5FF);
     if (color == const Color(0xFF0EA5E9)) return const Color(0xFFF2F8FF);
     if (color == AppColors.violet500) return const Color(0xFFF5F7FF);