hotfix(frontend/signup): Add missing createUser() call in password signup (#12287)

Requested by @0ubbe

Password signup was missing the backend `createUser()` call that the
OAuth callback flow already had. This caused `getOnboardingStatus()` to
fail/hang for new users whose backend record didn't exist yet, resulting
in an infinite spinner after account creation.

## Root Cause

| Flow | createUser() | getOnboardingStatus() | Result |
|------|-------------|----------------------|--------|
| OAuth signup | ✅ Called | ✅ Works | Redirects correctly |
| Password signup | ❌ Missing | ❌ Fails/hangs | Infinite spinner |

## Fix

Adds `createUser()` call in `signup/actions.ts` after session is set,
before onboarding status check — matching the OAuth callback pattern.
Includes error handling with Sentry reporting.

## Testing

- Create a new password account → should redirect without spinner
- OAuth signup unaffected (no changes to that flow)

Fixes OPEN-3023

---------

Co-authored-by: Lluis Agusti <hi@llu.lu>

autogpt_platform/frontend/src/app/(platform)/signup/actions.ts

@@ -1,10 +1,11 @@
 "use server";
 
+import { postV1GetOrCreateUser } from "@/app/api/__generated__/endpoints/auth/auth";
+import { getOnboardingStatus, resolveResponse } from "@/app/api/helpers";
 import { getServerSupabase } from "@/lib/supabase/server/getServerSupabase";
 import { signupFormSchema } from "@/types/auth";
 import * as Sentry from "@sentry/nextjs";
 import { isWaitlistError, logWaitlistError } from "../../api/auth/utils";
-import { getOnboardingStatus } from "../../api/helpers";
 
 export async function signup(
   email: string,
@@ -57,6 +58,17 @@ export async function signup(
       await supabase.auth.setSession(data.session);
     }
 
+    try {
+      await resolveResponse(postV1GetOrCreateUser());
+    } catch (createUserError) {
+      console.error("Error creating user during signup:", createUserError);
+      Sentry.captureException(createUserError);
+      return {
+        success: false,
+        error: "Failed to complete account setup. Please try again.",
+      };
+    }
+
     // Get onboarding status from backend (includes chat flag evaluated for this user)
     const { shouldShowOnboarding } = await getOnboardingStatus();
     const next = shouldShowOnboarding ? "/onboarding" : "/";