Export `TransactionKind` as a public type alias

[0xNgoo]

Summary

The Transaction type uses 'deposit' | 'withdrawal' inline, but the SDK does not expose a named TransactionKind type alias.

Why this matters

A named type alias makes consumer code a little more readable and avoids repeating the literal union in downstream apps.

Task

• Add a public TransactionKind type alias for 'deposit' | 'withdrawal'.
• Use the alias in the public transaction typing where it improves clarity without changing behavior.

Acceptance criteria

• TransactionKind is publicly exported.
• The existing Transaction typing remains compatible.

Testing notes

• Add or update focused tests for the behavior in this issue.
• Run bun test before opening the pull request.
• Add or update type tests that cover the new alias.

Contributor notes

• Keep the change narrowly scoped to the behavior described above.
• Use Bun for commands and TypeScript for any code changes.
• Avoid adding new runtime dependencies for this issue.

[shoaib050326]

Hi @0xNgoo Can you please review PR.