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/settings_screen.dart

@@ -9,7 +9,7 @@ class SettingsScreen extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: const Color(0xFFF8FAFC),
+      backgroundColor: AppColors.surfaceSecondary,
       body: SafeArea(
         child: Column(
           children: [
@@ -46,7 +46,7 @@ class SettingsScreen extends StatelessWidget {
           begin: Alignment.topLeft,
           end: Alignment.bottomRight,
           stops: [0, 1],
-          colors: [Color(0xFFFFFFFF), Color(0xFFF3F7FF)],
+          colors: [Color(0xFFFFFFFF), AppColors.surfaceInfoLight],
           transform: GradientRotation(35 * 3.14159 / 180),
         ),
         borderRadius: BorderRadius.circular(22),
@@ -97,7 +97,7 @@ class SettingsScreen extends StatelessWidget {
                         vertical: 4,
                       ),
                       decoration: BoxDecoration(
-                        color: const Color(0xFFF8FAFF),
+                        color: AppColors.surfaceTertiary,
                         borderRadius: BorderRadius.circular(10),
                         border: Border.all(color: const Color(0xFFDEE7F6)),
                       ),
@@ -144,7 +144,7 @@ class SettingsScreen extends StatelessWidget {
             child: _buildQuickActionCard(
               icon: Icons.people,
               iconColor: AppColors.blue600,
-              iconBg: const Color(0xFFF8FAFF),
+              iconBg: AppColors.surfaceTertiary,
               iconBorder: const Color(0xFFE6ECF7),
               title: '联系人',
               subtitle: '已添加 1 位：Toki',
@@ -279,9 +279,9 @@ class SettingsScreen extends StatelessWidget {
               width: 72,
               height: 32,
               decoration: BoxDecoration(
-                color: const Color(0xFFEAF3FF),
+                color: AppColors.surfaceInfo,
                 borderRadius: BorderRadius.circular(16),
-                border: Border.all(color: const Color(0xFFCFE1FB)),
+                border: Border.all(color: AppColors.borderQuaternary),
               ),
               child: const Center(
                 child: Text(
@@ -305,7 +305,7 @@ class SettingsScreen extends StatelessWidget {
       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: [