fix: add missing useEffect import in README example

[0x4tl4nt]

Summary

The "Using the connector" example in packages/web3-react-agw/README.md was using useEffect without importing it from React.
This causes a ReferenceError when copying the code.

Changes

• Added useEffect import to the example:

  import React, { useEffect } from 'react';

Reason

useEffect is a named export from React and must be explicitly imported.
This fixes the high-level issue reported in the review comment.

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PR-Codex overview

This PR focuses on updating the README.md file in the packages/web3-react-agw directory to include useEffect from React, indicating a potential enhancement in the usage of React hooks.

Detailed summary

• Added useEffect to the import statement from react in packages/web3-react-agw/README.md.

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Latest commit: b67d2f1

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PR Summary

Low Risk
Docs-only change to a README example import; no runtime or library behavior changes.

Overview
Fixes the packages/web3-react-agw/README.md “Using the connector” example by importing useEffect from React (import React, { useEffect } from 'react';), preventing a copy/paste ReferenceError in the sample code.

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