refactor(apps): 主题系统迁移至 ColorScheme + 扩展架构并支持 Dark Mode

apps/lib/features/todo/presentation/widgets/todo_drag_item.dart

@@ -30,7 +30,7 @@ class TodoDragItem extends StatelessWidget {
         borderRadius: BorderRadius.circular(AppRadius.md),
         child: Transform.scale(
           scale: 1.03,
-          child: SizedBox(width: 280, child: _buildDragFeedback()),
+          child: SizedBox(width: 280, child: _buildDragFeedback(context)),
         ),
       ),
       childWhenDragging: AnimatedOpacity(
@@ -44,18 +44,19 @@ class TodoDragItem extends StatelessWidget {
     );
   }
 
-  Widget _buildDragFeedback() {
+  Widget _buildDragFeedback(BuildContext context) {
+    final colorScheme = Theme.of(context).colorScheme;
     return Container(
       padding: const EdgeInsets.symmetric(
         horizontal: AppSpacing.md,
         vertical: AppSpacing.sm,
       ),
       decoration: BoxDecoration(
-        color: AppColors.white,
+        color: colorScheme.surface,
         borderRadius: BorderRadius.circular(AppRadius.md),
         boxShadow: [
           BoxShadow(
-            color: AppColors.slate400.withValues(alpha: 0.3),
+            color: colorScheme.shadow.withValues(alpha: 0.2),
             blurRadius: 12,
             offset: const Offset(0, 4),
           ),
@@ -65,10 +66,10 @@ class TodoDragItem extends StatelessWidget {
         todo.title,
         maxLines: 1,
         overflow: TextOverflow.ellipsis,
-        style: const TextStyle(
+        style: TextStyle(
           fontSize: 13,
           fontWeight: FontWeight.w600,
-          color: AppColors.slate700,
+          color: colorScheme.onSurfaceVariant,
         ),
       ),
     );