fix(sub): include xhttp mode in extra JSON for karing compatibility (#4365)

- Add mode to buildXhttpExtra() so clients reading xtra param
  (karing, etc.) receive the xhttp mode alongside other bidirectional
  SplitHTTP fields. Previously mode was only a flat URL param and was
  silently dropped when xtra was present.
- Add xhttp case to streamData() to strip acceptProxyProtocol and
  server-only fields (noSSEHeader, scMaxBufferedPosts,
  scStreamUpServerSecs, serverMaxHeaderBytes) from JSON sub configs.
- Sync frontend buildXhttpExtra() with the same mode addition.

Closes #4364

frontend/src/models/inbound.js

@@ -1595,6 +1595,10 @@ export class Inbound extends XrayCommonClass {
             });
         }
 
+        if (typeof xhttp.mode === 'string' && xhttp.mode.length > 0) {
+            extra.mode = xhttp.mode;
+        }
+
         const stringFields = [
             "sessionPlacement", "sessionKey",
             "seqPlacement", "seqKey",