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/calendar/ui/screens/calendar_event_detail_screen.dart

@@ -8,7 +8,7 @@ class CalendarEventDetailScreen extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: const Color(0xFFF8FAFC),
+      backgroundColor: AppColors.surfaceSecondary,
       body: SafeArea(
         child: Column(
           children: [
@@ -34,7 +34,7 @@ class CalendarEventDetailScreen extends StatelessWidget {
                 width: 36,
                 height: 36,
                 decoration: BoxDecoration(
-                  color: const Color(0xFFF8FAFF),
+                  color: AppColors.surfaceTertiary,
                   borderRadius: BorderRadius.circular(18),
                   border: Border.all(color: const Color(0xFFDEE7F6)),
                 ),
@@ -115,9 +115,9 @@ class CalendarEventDetailScreen extends StatelessWidget {
               width: 36,
               height: 36,
               decoration: BoxDecoration(
-                color: const Color(0xFFF8FAFF),
+                color: AppColors.surfaceTertiary,
                 borderRadius: BorderRadius.circular(12),
-                border: Border.all(color: const Color(0xFFDCE5F4)),
+                border: Border.all(color: AppColors.borderTertiary),
               ),
               child: const Icon(
                 LucideIcons.pencil,
@@ -190,11 +190,11 @@ class CalendarEventDetailScreen extends StatelessWidget {
             const SizedBox(width: 10),
             _buildColorOption(const Color(0xFF8B5CF6)),
             const SizedBox(width: 10),
-            _buildColorOption(const Color(0xFF10B981)),
+            _buildColorOption(AppColors.success),
             const SizedBox(width: 10),
-            _buildColorOption(const Color(0xFFF59E0B)),
+            _buildColorOption(AppColors.warning),
             const SizedBox(width: 10),
-            _buildColorOption(const Color(0xFFEF4444)),
+            _buildColorOption(AppColors.error),
           ],
         ),
       ],
@@ -237,7 +237,7 @@ class CalendarEventDetailScreen extends StatelessWidget {
           decoration: BoxDecoration(
             color: const Color(0xFFFDFEFF),
             borderRadius: BorderRadius.circular(12),
-            border: Border.all(color: const Color(0xFFDCE5F4)),
+            border: Border.all(color: AppColors.borderTertiary),
           ),
           child: const Column(
             crossAxisAlignment: CrossAxisAlignment.start,
@@ -262,7 +262,7 @@ class CalendarEventDetailScreen extends StatelessWidget {
     return Container(
       height: 80,
       padding: const EdgeInsets.all(16),
-      color: const Color(0xFFF8FAFC),
+      color: AppColors.surfaceSecondary,
       child: Row(
         children: [
           Container(
@@ -271,7 +271,7 @@ class CalendarEventDetailScreen extends StatelessWidget {
             decoration: BoxDecoration(
               color: Colors.white,
               borderRadius: BorderRadius.circular(18),
-              border: Border.all(color: const Color(0xFFE2E8F0)),
+              border: Border.all(color: AppColors.borderSecondary),
             ),
             child: const Icon(
               LucideIcons.plus,