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/inbound-form-adapter.test.ts

@@ -142,6 +142,17 @@ describe('formValuesToWirePayload', () => {
     expect(payload.streamSettings).toBe('');
   });
 
+  it('emits empty sniffing for mtproto (mtg-served, not Xray)', () => {
+    const values = rawInboundToFormValues({
+      ...vlessRow,
+      protocol: 'mtproto',
+      settings: { fakeTlsDomain: 'www.cloudflare.com', secret: 'ee00' },
+    });
+    const payload = formValuesToWirePayload(values);
+    expect(payload.protocol).toBe('mtproto');
+    expect(payload.sniffing).toBe('');
+  });
+
   it('omits nodeId when null', () => {
     const values = rawInboundToFormValues({ ...vlessRow, nodeId: null });
     const payload = formValuesToWirePayload(values);