Link production sign-in to waitlist

[leoisadev1]

Summary

• replace the production sign-in dead-end copy with a Join the waitlist link
• preserve the typed email when moving from sign-in to /sign-up
• update the auth smoke guard to require the production waitlist path

Verification

• bun run lint
• bun run check-types
• bun run build

Note

• full smoke still requires the local runtime/env setup and currently fails in this clean worktree on existing local runtime/dashboard assertions, so this change used the narrow static gates plus the updated source guard.