feat(mtproto): add domain-fronting and essential mtg options

Expose mtg's [domain-fronting] section (ip/port/proxy-protocol) plus
proxy-protocol-listener, prefer-ip, and debug on MTProto inbounds. Each
key is written to the generated mtg-<id>.toml only when set, so mtg's own
defaults apply otherwise. The instance fingerprint now covers these
fields, so editing an option restarts the sidecar.

Since MTProto is mtg-served (not Xray), sniffing does not apply: hide the
Sniffing tab and the Advanced sniffing sub-editor, drop it from the
Advanced "All" JSON view, and emit empty sniffing in the wire payload,
all gated by a new canEnableSniffing predicate.

x-ui.sh

@@ -923,7 +923,6 @@ show_mtproto_status() {
             echo -e "mtproto inbound ${id} (${bind}): ${red}Not Running${plain}"
         fi
     done
-    echo -e "  ${yellow}mtg logs:${plain} journalctl -u x-ui --no-pager -n 200 | grep -i mtproto"
 }
 
 firewall_menu() {