fix(sub): re-add xhttp mode to extra JSON for Karing (#5446)

Regression of #4364. Karing parses the `extra` JSON and ignores the
flat `mode=` param, so when extra was present without `mode` it stored
the transport with no mode and the handshake failed. The `mode` field
that #4365 added to buildXhttpExtra was dropped during the share-link
refactor; restore it in both the backend and frontend generators.

frontend/src/lib/xray/inbound-link.ts

@@ -47,6 +47,10 @@ function buildXhttpExtra(xhttp: XHttpStreamSettings | undefined): Record<string,
   if (!xhttp) return null;
   const extra: Record<string, unknown> = {};
 
+  if (typeof xhttp.mode === 'string' && xhttp.mode.length > 0) {
+    extra.mode = xhttp.mode;
+  }
+
   if (typeof xhttp.xPaddingBytes === 'string' && xhttp.xPaddingBytes.length > 0) {
     extra.xPaddingBytes = xhttp.xPaddingBytes;
   }