Add getAdmin and transferJarOwnership to the client

[Abioladory123]

Summary

TipSplitterClient covers getJar, getToken, createJar, tip and
updateSplits. The contract has since gained get_admin and
transfer_jar_ownership, and neither is wrapped.

Ownership transfer in particular is a state-changing call that consumers have to
hand-roll against the raw contract today, which is precisely the work this
package exists to remove.

Acceptance criteria

• getAdmin() reads the admin address, following the existing view pattern.
• transferJarOwnership(params, opts) follows the existing invoke pattern,
  including signing options and typed error mapping.
• Both are documented in the client section of the README.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/novatip-sdk.git
cd novatip-sdk
git remote add upstream https://github.com/Novatip/novatip-sdk.git
npm install

Create a branch for this issue:

git checkout -b feat/client-admin-and-transfer

Suggested commit message:

feat: wrap get_admin and transfer_jar_ownership

Run npm run lint, npm run typecheck, and npm run build before pushing, then open a pull request from your fork against main and link this issue. See the contributing guide for the full workflow.