feat(tasks): add color_code_dark to task groups and enhance task list display

- Introduced color_code_dark property to task groups for improved theming support.
- Updated task list components to utilize color_code_dark based on the current theme mode.
- Enhanced empty state handling in task list to dynamically create an unmapped group for better user experience.
- Refactored task management slice to support dynamic group creation for unmapped tasks.

worklenz-backend/src/controllers/tasks-controller-base.ts

@@ -16,6 +16,7 @@ export interface ITaskGroup {
   start_date?: string;
   end_date?: string;
   color_code: string;
+  color_code_dark: string;
   category_id: string | null;
   old_category_id?: string;
   todo_progress?: number;