feat(contract): execute with nonce check and ContractError

[Agbasimere]

Summary

Closes #30

• Add ContractError enum and use it in the account contract instead of raw panics.
• Implement execute() with nonce validation, contract invoke, and return value.
• Replace panics with ContractError in initialize and get_owner.
• Add tests for nonce validation and cross-contract execution.

Type of change

• New feature (nonce-gated execute)
• Refactor (error handling via ContractError)

Checklist

• Contract builds
• Tests added/updated