9. Add `lib/utils/validation.ts` form validation

[DeFiVC]

What: Add form validation utilities for common fields (email, address, name).

Why: Forms need validation (required fields, format checks, length limits). A utility module centralizes these.

Scope: Add validators for: required, minLength, maxLength, stellarAddress, displayName. Return error messages.

Acceptance criteria:

• Validators work correctly
• Error messages are clear
• Composable validators work
• Edge cases are handled

Technical context: src/lib/utils/validation.ts — validation utils. src/app/(auth)/onboarding/page.tsx — onboarding form.

---