#114 Cannot pause or cancel running workflows with activity workers

Currently there is no mechanism to pause or cancel a running workflow, especially those using activity workers. Required functionality: 1. API endpoint to pause a running workflow 2. API endpoint to cancel/abort a running workflow 3. Activity workers should respect pause/cancel signals 4. Frontend UI controls for pause/cancel actions 5. Proper state management for paused workflows (resumable) 6. Clean termination of in-flight activity tasks on cancel This is critical for operational control of long-running workflows.