Forward client IPs transparently to backends

Run flowguard setup to choose how a proxy exposes the validated client address to a same-host Linux backend. Transparent forwarding preserves the client IP as the TCP source when the client and backend use the same IP family. The panel shows the selected mode and any IPv4/IPv6 address pairs.

Requires FlowGuard Proxy v0.15.0 or newer. Restart the proxy after changing networking settings.