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-frontend/src/hooks/useTaskSocketHandlers.ts

@@ -855,10 +855,11 @@ export const useTaskSocketHandlers = () => {
           // For priority grouping, use priority field (which contains the priority UUID)
           groupId = data.priority;
         } else if (grouping === 'phase') {
-          // For phase grouping, use phase_id
-          groupId = data.phase_id;
+          // For phase grouping, use phase_id, or 'Unmapped' if no phase_id
+          groupId = data.phase_id || 'Unmapped';
         }
 
+
         // Use addTaskToGroup with the actual group UUID
         dispatch(addTaskToGroup({ task, groupId: groupId || '' }));