Issue #7: Profile validation logic was embedded in page component.
Changes:
- Create utils/validation.ts with shared validation functions:
- validateEmail: email format validation
- validateTelegram: handle format with @ prefix
- validateSignal: username length validation
- validateNostrNpub: bech32 format validation
- validateProfileFields: combined validation
- Update profile/page.tsx to use shared validation
- Both frontend and backend now read validation rules from
shared/constants.json for consistency
