Menu bar close must kill all app instances

[techmore]

Issue: When the NmapUI menu bar icon is closed, the background Flask application (running on port 9000) continues to run. This is unacceptable because users expect the entire application to terminate when they close the menu bar.\n\nExpected Behavior:\n- Closing the menu bar application should terminate all associated background processes, including the Flask server and any scan processes.\n\nCurrent Behavior:\n- The menu bar closes but the Flask app remains active, consuming resources and potentially causing confusion.\n\nSuggested Solution:\n- Implement a mechanism where the menu bar application monitors the main app process and terminates it upon exit.\n- Alternatively, use a parent-child process relationship so that when the parent (menu bar) exits, the child (Flask app) is automatically terminated.\n- Ensure proper cleanup of resources and child processes on exit.