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/memory_screen.dart

@@ -21,7 +21,7 @@ class _MemoryScreenState extends State<MemoryScreen> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: const Color(0xFFF8FAFC),
+      backgroundColor: AppColors.surfaceSecondary,
       body: SafeArea(
         child: Column(
           children: [
@@ -55,7 +55,7 @@ class _MemoryScreenState extends State<MemoryScreen> {
       decoration: BoxDecoration(
         color: AppColors.white,
         borderRadius: BorderRadius.circular(16),
-        border: Border.all(color: const Color(0xFFE2E8F0)),
+        border: Border.all(color: AppColors.borderSecondary),
       ),
       child: Column(
         children: [
@@ -129,7 +129,7 @@ class _MemoryScreenState extends State<MemoryScreen> {
         height: 74,
         padding: const EdgeInsets.all(12),
         decoration: BoxDecoration(
-          color: const Color(0xFFF8FAFF),
+          color: AppColors.surfaceTertiary,
           borderRadius: BorderRadius.circular(12),
           border: Border.all(color: const Color(0xFFE8EDF7)),
         ),
@@ -139,7 +139,7 @@ class _MemoryScreenState extends State<MemoryScreen> {
               width: 32,
               height: 32,
               decoration: BoxDecoration(
-                color: const Color(0xFFE8F3FF),
+                color: AppColors.surfaceInfo,
                 borderRadius: BorderRadius.circular(10),
               ),
               child: Icon(item.icon, size: 16, color: AppColors.blue500),