Fix: add return button to SignTransaction component

[Meesut0]

Fixed the SignTransaction example in
abstract-global-wallet/agw-client/actions/signTransaction.mdx.

The component previously declared a React function but did not return any JSX, making the example incomplete and not directly usable as a copy-paste snippet.

This change adds a simple <button> element that calls signTransaction, making the example consistent with the SendTransaction example and other examples mentioned in PR: #198.

[cursor]

PR Summary

Low Risk
Low risk documentation-only change that only affects an MDX example snippet. The main risk is minor: the updated snippet could be copied into apps, but it only adds a simple button that triggers the existing signTransaction call.

Overview
Fixes the abstract-global-wallet/agw-client/actions/signTransaction.mdx usage example by adding a returned JSX <button> wired to signTransaction, making the snippet runnable/copy-pasteable instead of defining a component that never renders anything.

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