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.

frontend/src/test/golden/fixtures/inbound/mtproto-domain-fronting.json

@@ -0,0 +1,15 @@
+{
+  "protocol": "mtproto",
+  "settings": {
+    "fakeTlsDomain": "www.cloudflare.com",
+    "secret": "ee0123456789abcdef0123456789abcdef7777772e636c6f7564666c6172652e636f6d",
+    "proxyProtocolListener": true,
+    "preferIp": "prefer-ipv4",
+    "debug": true,
+    "domainFronting": {
+      "ip": "127.0.0.1",
+      "port": 9443,
+      "proxyProtocol": true
+    }
+  }
+}