feat: implement Stellar wallet address validation

[AugistineCreates]

Closes #196

Adds robust validation for Stellar wallet addresses before they are stored or used in blockchain operations. This prevents malformed or incompatible addresses from reaching transaction and ownership workflows.

Tasks Completed

✅ Implemented validateStellarAddress utility leveraging stellar-sdk's StrKey.isValidEd25519PublicKey for robust validation and checksum checking.
✅ Integrated the validation utility into authentication (lib/stellar-auth.ts), transaction (lib/stellar.ts), and ownership workflows.
✅ Handled valid, invalid, and edge cases gracefully by returning clear error feedback to avoid silent failures.
✅ Added a comprehensive suite of unit tests covering the validation utility.
Acceptance Criteria Met

Only valid Stellar addresses are accepted; malformed addresses are immediately rejected with standardized errors.
Checks are highly reusable and enforce consistent wallet address requirements across all services.
Database integrity is completely preserved by intercepting invalid inputs before storage or on-chain submission.

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