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/lib/xray/protocol-capabilities.ts

@@ -50,6 +50,12 @@ export function canEnableStream(values: { protocol: string }): boolean {
   return STREAM_PROTOCOLS.includes(values.protocol);
 }
 
+// mtproto is served by an external mtg process, not Xray, so the Xray sniffing
+// block does not apply to it. Every other inbound supports sniffing.
+export function canEnableSniffing(values: { protocol: string }): boolean {
+  return values.protocol !== 'mtproto';
+}
+
 // Vision seed applies only when XTLS Vision (TCP/TLS) flow is selected
 // AND at least one VLESS client uses the vision flow. Excludes UDP variant.
 export function canEnableVisionSeed(values: CapabilityVlessSlice): boolean {