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;
   }

internal/sub/service.go

@@ -1657,6 +1657,10 @@ func buildXhttpExtra(xhttp map[string]any) map[string]any {
 	}
 	extra := map[string]any{}
 
+	if mode, ok := xhttp["mode"].(string); ok && len(mode) > 0 {
+		extra["mode"] = mode
+	}
+
 	if xpb, ok := xhttp["xPaddingBytes"].(string); ok && len(xpb) > 0 {
 		extra["xPaddingBytes"] = xpb
 	}

internal/sub/service_test.go

@@ -369,8 +369,10 @@ func TestBuildXhttpExtra_IncludesClientSideFieldsWhenPresent(t *testing.T) {
 			t.Fatalf("extra missing %q: %#v", key, extra)
 		}
 	}
-	if _, ok := extra["mode"]; ok {
-		t.Fatalf("mode should stay as a top-level query parameter, got extra %#v", extra)
+	// mode rides inside extra (in addition to the flat param) so clients
+	// that only read the extra JSON keep the xhttp mode (#5446).
+	if extra["mode"] != "packet-up" {
+		t.Fatalf("extra[mode] = %#v, want packet-up", extra["mode"])
 	}
 
 	headers, ok := extra["headers"].(map[string]any)