[ HumanjavaEnterprises/nostr-dm-magiclink-utils ] fix: enforce key validation in all environments, implement message verification

HIGH fixes:
- NIP-04 key format validation now runs unconditionally (was skipped
  outside production — malformed keys accepted in dev/staging)
- verifyMessage() now performs actual Schnorr signature verification
  via verifyEvent() (was a no-op stub that always returned true)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/HumanjavaEnterprises/nostr-dm-magiclink-utils/commit/b12450e3e9bb84518fc1a811624bf3d4763cb05c