Improve sign-up validation error layout

**Problem**
Sign-up page error states use display:none to hide validation fields instead of removing them from layout flow, leaving large empty gaps in the form. Flagged in the 6/29 internal check-in — agreed not to reopen with Rob mid-PR, tracked here instead so it's on record before Vini (likely) flags it externally.

**Scope**
Review current hidden-field implementation for sign-up validation
Redesign error-state layout so hidden/inactive fields don't leave gaps
Confirm existing validation logic is unaffected

**Acceptance criteria**

Error states render without large empty gaps
Hidden/inactive validation content doesn't disrupt spacing
Validation behavior unchanged from current implementation
Layout reviewed against design system bar

**Notes**

Non-blocking — doesn't hold up current sign-up PR
Originates from Rob's display:none approach; logged proactively per 6/29 decision

Start by locating the sign-up page's validation implementation and reviewing how hidden fields are handled in error states. Compare the layout with the design system bar, then verify that error states have no large gaps and that existing validation behavior is unchanged.

Written by the indexing model from the issue text.