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.

web/translation/en-US.json

@@ -504,7 +504,13 @@
         "encryptionMethod": "Encryption method",
         "fakeTlsDomain": "FakeTLS domain (SNI)",
         "mtprotoSecret": "Secret",
-        "mtprotoHint": "MTProto is served by a separate mtg process, not Xray. Stream settings and clients do not apply here — share the link below with Telegram.",
+        "mtgDomainFrontingIp": "Domain fronting IP",
+        "mtgDomainFrontingPort": "Domain fronting port",
+        "mtgDomainFrontingProxyProtocol": "Domain fronting PROXY protocol",
+        "mtgDomainFrontingHint": "Where mtg sends non-Telegram traffic — e.g. your NGINX fake site. Leave the IP empty to use the FakeTLS domain via DNS; default port is 443.",
+        "mtgProxyProtocolListener": "Accept PROXY protocol (listener)",
+        "mtgPreferIp": "IP preference",
+        "mtgDebug": "Debug logging",
         "visionTestseed": "Vision testseed",
         "version": "Version",
         "udpIdleTimeout": "UDP idle timeout (s)",