fix(client): prevent wireguard lifecycle reinitialization

[LaurenceJJones]

Community Contribution License Agreement

By creating this pull request, I grant the project maintainers an unlimited,
perpetual license to use, modify, and redistribute these contributions under any terms they
choose, including both the AGPLv3 and the Fossorial Commercial license terms. I
represent that I have the right to grant this license for all contributed content.

Description

• make WireGuard client interface setup idempotent on repeated config updates
• serialize periodic bandwidth-check start/stop to avoid shutdown races
• cleanly close the native UAPI listener during client shutdown

How to test?

Internal code fixes.