enhancement: export pure transaction builders from SDK

[devmocrea]

Description

Add browser-agnostic transaction construction functions to packages/sdk so developers can generate, sign, and broadcast transactions from server-side NodeJS environments.

Technical Specification

• In packages/sdk/src/index.ts, add functions to build unsigned transactions using makeContractCall.
• Allow passing private keys directly to these functions for server-side transaction signing.

Acceptance Criteria

• Developers can import these utilities and sign transactions inside pure backend environments.