feat: tunnel host and port in the panel settings, ufw kept in sync

The direct tunnel port (8446 by default) and the host agents dial for
it can be set in Settings. The server moves its listeners at once and
agents pick the address up from the panel: on connect and with every
poll, unless TUNNEL_ADDR / TUNNEL_PORT pin it locally.

ufw: run with a C locale (the output is translated), do not adopt rules
that existed before the panel, skip loopback-only listeners, re-open
what saved forwards need on start and when the integration is turned
on, move the shared tunnel rule with the port and drop it when no
direct forward is left. The last ufw error is shown in Settings.

The forwards page shows the real tunnel and panel ports.

deploy/lpra-server.service

@@ -19,6 +19,13 @@
 # the firewall:  ufw allow 8446/tcp ; ufw allow 8446/udp
 # (The "http" forward mode and terminal/files go through nginx :8445 and need
 # nothing extra.) Override with LPRA_TUNNEL_ADDR, or set it empty to disable.
+# The port, and the host agents dial for it, can also be changed in the panel
+# settings; agents pick the new address up from the panel by themselves.
+#
+# ufw integration (the panel opening forwarded ports itself) needs the server
+# to run as root, and ufw's state must be writable despite ProtectSystem=strict
+# below — otherwise "ufw status" works but "ufw allow" fails:
+#   ReadWritePaths=/etc/ufw /run
 
 [Unit]
 Description=LPRA remote access host (admin panel + agent rendezvous)