fix(auth): show registration-capacity block as a persistent inline notice with recovery

[posthog]

Problem

• Sign-up dies silently on the capacity block: the 409 TENANT_REGISTRATION_BLOCKED (no cluster capacity for a new tenant) showed only a destructive toast that faded, leaving email, domain, terms, and the enabled submit button untouched — the UI told the user to retry while nothing about the attempt could succeed.
• The live availability check had the same blind spot: it folded the 409 into a silent 'error' status that rendered no message, so the user could not learn registration was closed before pressing Create Account.
• Nothing measured it. The project has no registration_blocked event, so the block emitted no telemetry and was only visible in session replay.
• Two smaller issues in the same path: /auth/signup (the primary CTA target) was never prefetched, and two /auth paths were hard-coded instead of using the route registry.

Changes

• Route the capacity block through the form's persistent inline domain-status slot (domainStatusMessage, warning variant) — where every other availability outcome already lives — and keep submit disabled while it holds. Clears on edit so the live check can re-evaluate.
• useDomainAvailability now reports the 409 honestly (new registration-blocked status + backend message) instead of swallowing it as 'error'.
• Add pushRegistrationBlocked() / registration_blocked funnel event so the block finally shows up in the data.
• Prefetch routes.auth.signup; swap the two hard-coded /auth paths for the registry entries.
• Extract a shared isTenantRegistrationBlocked(response) helper so the live check and the submit-time re-check read the same 409 envelope.

The capacity condition and the exact "try again in about 10 minutes" wording come from the backend in another repo; the frontend uses a fallback string only when the backend sends none.

Notes

• Scope is the four auth files named in the report plus a shared helper and the analytics event.
• npm run type-check and npm run lint:biome pass.

Agent context

• Considered a form-level banner prop in the shared core lib (cleaner altitude for an environment condition), but that is an external repo and out of scope; the inline domain slot is the surface the report specifies and matches the existing availability UX.

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