Sign-in modal: register via Auth0 native signup, not Cloud's wizard

[jvorcak]

Summary

The header sign-in modal's "Create a free account" link is hardcoded to cloud.redpanda.com/sign-up — Cloud's full onboarding wizard, which provisions a trial org + resources even for a docs-only sign-in.

This mirrors the "Continue" button's existing pattern (modalCta.href = '/login?disclosed=1&return_to=...') for the signup case: point at docs-site's /login?disclosed=1&signup=1&return_to=..., which resolves (as of docs-site#212) to Auth0's native screen_hint=signup screen on the docs Auth0 client, instead of Cloud's wizard.

Depends on

docs-site#212 must land first (or at least the signup=1 fast path in docs-login.mjs) for /login?signup=1 to resolve correctly — until then this link 404s/falls through to the plain interstitial.

Why draft

Base is feature/ask-ai-agent-sdk (PR #395), not main — this only makes sense once that lands. Also: the underlying assumption (Auth0 native signup doesn't trigger Cloud's trial-resource provisioning) is still being verified on the docs-site preview; flagging here in case that turns out false and this needs to be reverted/rethought.

Test plan

• docs-site#212 (or its signup=1 support) is merged into this feature branch's target environment
• On a preview build, click "Create a free account" from the header sign-in modal
• Confirm it lands on Auth0's native "Sign up" screen (not cloud.redpanda.com)
• Confirm return_to still round-trips correctly after signup completes

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[JakeSCahill]

Pushed a follow-up commit as part of a cross-repo review of this PR alongside docs-site #181/#212 and cloudv2 #29334 (findings verified against live code):

Reverted this PR's core change (screen_hint=signup fast path for "Create a free account"). Confirmed by direct reproduction: any Auth0-native signup on the docs Auth0 client (which requires org membership) produces a permanently org-less account — login_error=org_setup_incomplete forever, since verifying the emailed address does not create an organization (only Cloud's real onboarding wizard does). #212 on docs-site (the dependency this PR names) never verified its own core assumption and should be treated the same way.

The underlying waste concern (Cloud's wizard provisioning a trial cluster nobody asked for) has a real fix that doesn't break login: cloudv2's signUpSource/backoffice-worker work (currently #29334) tags docs-referred signups and skips just the welcome-cluster provisioning, while still creating a real org through the normal pipeline. "Create a free account" now goes back to cloud.redpanda.com/sign-up with the existing UTM tagging, which is exactly the signal that mechanism reads.

Two more contained fixes while in this file:

• returnTo() was dropping window.location.hash — a reader on a #section-anchor who signed in landed back at the top of the page instead of where they were. Now included; docs-site's safeReturnTo() already accepts it as-is.
• The org_setup_incomplete/access_denied toast copy ("click the verification link, then sign in again") can't actually fix a missing-org account — corrected to match the same fix just pushed to docs-site's docs-login.mjs (point at finishing Cloud setup, not the inbox). Currently unreachable dead code under the current docsLoginError redirect design (server-side interstitial handles all login errors now), but kept in sync per this file's own comment in case that changes.

No test harness exists for this file (node --check syntax-verified; no lint/test script wired to it in this repo).

Not fixed here, flagged only: Auth0's own hosted Universal Login page still renders its native "Sign up" link independent of anything in docs-ui — reachable via "Continue" → Auth0's hosted login → its own signup link — which produces the same permanently org-less account. That's Auth0 tenant/connection config, out of reach from this repo.