fix(react-widget): drop the stranded 'use client' directive

Parcel's library build concatenates modules, so the directive at the top
of DocsGPTWidget.tsx landed a thousand lines into dist/module.js and
dist/main.js. webpack rejects a 'use client' that is not the first
expression in the file, which broke every Next.js consumer of
docsgpt-react:

  x The "use client" directive must be placed before other expressions.
    1168 | 'use client';

The directive could not do its job from inside a bundle anyway. A React
Server Components boundary has to be declared by the importing module,
so consumers wrap the widget in their own client component instead.

extensions/react-widget/src/components/DocsGPTWidget.tsx

@@ -1,4 +1,3 @@
-'use client';
 import React, { useRef } from 'react';
 import DOMPurify from 'dompurify';
 import styled, { keyframes, css } from 'styled-components';