fix(frontend): revalidate layout after email/password login (#12285)

Requested by @ntindle

After logging in with email/password, the page navigates but renders a
blank/unauthenticated state (just logo + cookie banner). A manual page
refresh fixes it.

The `login` server action calls `signInWithPassword()` server-side but
doesn't call `revalidatePath()`, so Next.js serves cached RSC payloads
that don't reflect the new auth state. The OAuth callback route already
does this correctly.

**Fix:** Add `revalidatePath(next, "layout")` after successful login,
matching the OAuth callback pattern.

Closes SECRT-2059

autogpt_platform/frontend/src/app/(platform)/login/useLoginPage.ts

@@ -21,8 +21,14 @@ export function useLoginPage() {
   const [showNotAllowedModal, setShowNotAllowedModal] = useState(false);
   const isCloudEnv = environment.isCloud();
 
-  // Get redirect destination from 'next' query parameter
-  const nextUrl = searchParams.get("next");
+  // Get redirect destination from 'next' query parameter.
+  // Only allow relative paths to prevent open redirect attacks
+  // (e.g., /login?next=https://phishing.site).
+  const rawNext = searchParams.get("next");
+  const nextUrl =
+    rawNext && rawNext.startsWith("/") && !rawNext.startsWith("//")
+      ? rawNext
+      : null;
 
   useEffect(() => {
     if (isLoggedIn && !isLoggingIn) {
@@ -93,8 +99,11 @@ export function useLoginPage() {
         throw new Error(result.error || "Login failed");
       }
 
-      // Prefer URL's next parameter, then use backend-determined route
-      router.replace(nextUrl || result.next || "/");
+      // Use full page navigation to ensure middleware processes the new auth cookies.
+      // router.replace() does a soft navigation where the cookie store may not
+      // immediately reflect cookies set by the server action, causing a blank page.
+      // This matches the OAuth flow which also uses window.location.href.
+      window.location.href = nextUrl || result.next || "/";
     } catch (error) {
       toast({
         title: