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/zh-CN.json

@@ -503,7 +503,13 @@
         "encryptionMethod": "加密方法",
         "fakeTlsDomain": "FakeTLS 域名 (SNI)",
         "mtprotoSecret": "密钥",
-        "mtprotoHint": "MTProto 由独立的 mtg 进程提供服务，而非 Xray。传输设置和客户端在此不适用——请将下方链接分享到 Telegram。",
+        "mtgDomainFrontingIp": "域前置 IP",
+        "mtgDomainFrontingPort": "域前置端口",
+        "mtgDomainFrontingProxyProtocol": "域前置 PROXY 协议",
+        "mtgDomainFrontingHint": "mtg 转发非 Telegram 流量的目标——例如你的 NGINX 伪装站点。留空 IP 则通过 DNS 解析 FakeTLS 域名；默认端口为 443。",
+        "mtgProxyProtocolListener": "接受 PROXY 协议（监听器）",
+        "mtgPreferIp": "IP 优先级",
+        "mtgDebug": "调试日志",
         "visionTestseed": "Vision testseed",
         "version": "版本",
         "udpIdleTimeout": "UDP 空闲超时 (s)",