Auto-detect public IP on pivpn -a to handle dynamic IPs

[diminished-5th]

Auto-detect Public IP in pivpn -a

I understand PiVPN is in maintenance mode now, but this could be a quick win for users experiencing a common issue.

The Problem:
When your ISP assigns a new public IP, pivpn -a generates client configs with the stale endpoint stored in setupVars.conf from initial setup.

Related Issue:
See #1857 — user's VPN breaks after a few days.

Current Workaround:

• Manually edit setupVars.conf with new IP
• Manually edit all existing client configs to update endpoints
• Or set up DDNS

Proposed Solution:
Have pivpn -a auto-detect the current public IP before generating configs (similar to what install.sh already does with curl -sSf https://checkip.amazonaws.com). This way new configs always have the correct endpoint.

[orazioedoardo]

Seems a good idea