The sign-in return no longer prefetches the target route (D-366)

Measured in production with 300 ms latency, router.prefetch loaded only the route tree while auth/me was in flight, and the replace still fetched the route payload after auth/me: no gain on the critical path, one to three extra requests per sign-in return.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Web/autopilot-monitor-web/components/landing/AuthGate.tsx

@@ -2,7 +2,7 @@
 
 import { useAuth } from "../../contexts/AuthContext";
 import { useRouter } from "next/navigation";
-import { useEffect, useRef } from "react";
+import { useEffect } from "react";
 import type { Route } from "next";
 import { landingTarget } from "../../lib/landingTarget";
 import { consumePostLoginReturnUrl, peekPostLoginReturnUrl } from "../../lib/postLoginReturn";
@@ -10,7 +10,6 @@ import { portalHandoverUrl, shouldCrossOriginToPortal } from "../../lib/hostRout
 import { consumePendingRehome, getSelectedAuthApp, legacyConfigured, switchAuthApp, tryBeginRehome } from "../../lib/authApp";
 import { activeAuthApp } from "../../lib/msalConfig";
 import { trackEvent } from "../../lib/appInsights";
-import { trustedRoute } from "../../lib/routes";
 import { AUTH_PENDING_CLASS, hasAuthHint, holdsAuthOverlay } from "../../lib/authHint";
 
 /**
@@ -23,16 +22,6 @@ export function AuthGate() {
   const { isAuthenticated, isLoading, user, isActivationPending } = useAuth();
   const router = useRouter();
 
-  // Load the target's route payload while auth/me is still in flight, so the replace below commits
-  // without a further round trip. Same-origin only: on the public host the target lives on portal.
-  const targetPrefetched = useRef(false);
-  useEffect(() => {
-    if (!isAuthenticated || targetPrefetched.current || shouldCrossOriginToPortal()) return;
-    targetPrefetched.current = true;
-    const returnUrl = peekPostLoginReturnUrl();
-    router.prefetch(returnUrl ? trustedRoute(returnUrl) : "/dashboard");
-  }, [isAuthenticated, router]);
-
   useEffect(() => {
     if (isAuthenticated && !isLoading && user) {
       // Dual app-reg window: the sign-in that just completed ran on the other app than the