fix: cleanup MITM hosts on exit

[manhdzzz]

Summary

Fix a shutdown bug where MITM hosts entries could remain after exiting 9Router.

Root cause

The CLI force-kills the Next.js process with SIGKILL before its cleanup handlers can run, so DNS/hosts cleanup may never execute.

Changes

• Add cleanupMitmHosts.js for CLI-level hosts cleanup.
• Run hosts cleanup before terminating child processes.
• Gracefully send SIGTERM to the Next.js process before falling back to SIGKILL.
• Ensure update/restart paths also clean up hosts entries.

Result

• Temporary MITM hosts entries are reliably removed when exiting, restarting, or updating the CLI.
• The Error:
  "There was an unexpected issue setting up your account."
  when launching Antigravity without 9router enabled has been fixed.