feat: add task cancellation feature (task-management)

- Backend: Add API endpoint to cancel tasks and mark status as cancelled
- Frontend: Add cancel button to task management UI (visible only for cancellable tasks)
- Process: Implement cancellation check during generation to abort in-progress tasks
- UI: Update status display styles with a visual indicator for the cancelled state

backend/app/routers/tasks.py

@@ -21,3 +21,19 @@ def get_project_tasks(project_id: str, session: Session = Depends(get_session)):
     # Filter only recent or active tasks if list is too long?
     # For now return all, maybe limit 20
     return tasks[:20]
+
+@router.post("/{task_id}/cancel", response_model=TaskRead)
+def cancel_task(task_id: str, session: Session = Depends(get_session)):
+    task = session.get(Task, task_id)
+    if not task:
+        raise HTTPException(status_code=404, detail="Task not found")
+    
+    if task.status in ["completed", "failed", "cancelled"]:
+        return task
+        
+    task.status = "cancelled"
+    task.message = "Task cancelled by user"
+    session.add(task)
+    session.commit()
+    session.refresh(task)
+    return task