Align signup validation with shared async validation helpers

[Abidoyesimze]

Summary

Signup uses form-specific validation patterns that duplicate shared helpers.

Affected files

• src/app/(auth)/signup/page.tsx
• src/lib/form-validation.ts

Tasks

• Replace bespoke signup validation with form-validation utilities.
• Remove duplicated regex/rules where shared helpers exist.
• Keep existing validation rules and user-facing messages.

Acceptance criteria

• Scope is limited to the files listed (or agreed alternatives in the PR).
• Change is verifiable: tests, screenshots, or written QA steps in the PR.
• No behavior regressions in related user flows.
• No new duplicate abstractions without a one-line rationale in the PR.

Suggested labels

cleanup, refactor, good first issue

Notes

Standalone cleanup issue from CLEANUP_ISSUES_2026-05.md. Complements the audit queue and product backlog.