Externalize React/ReactDOM and add en-US locale fallback to support Silverstripe 6

[tomstgeorge]

Summary

This PR resolves two critical issues that block the SEO module's analysis features in Silverstripe 6 environments.

The Problem

1. React Hook Conflict (Error #321): The module was bundling its own copy of React/ReactDOM into main.js. In Silverstripe 6 (React 18), this causes an "Invalid hook call" crash because the component tries to use a different React context than the one provided by the CMS admin.
2. Missing en-US Locale: The module hardcodes a fallbackLng: 'en-US' in i18n.js, but the package only contained an en folder, leading to 404 errors that prevent the analysis field from initializing.

The Fix

• Webpack Externals: Updated webpack.config.js to treat react, react-dom, and react-dom/client as external dependencies.
• Global Variable Mapping: Mapped these externals to the specific global variables exposed by silverstripe-admin in SS6:
  • react -> React
  • react-dom -> ReactDom
  • react-dom/client -> ReactDomClient
• Fixed Locales: Added the missing en-US translation files to the source to resolve 404 errors.
• Updated Dist: Compiled a fresh production build of main.js with these mappings applied.

Verification

• Tested on Silverstripe 6.1.
• Confirmed the "Minified React error #321" is resolved.
• Confirmed the Keyword Analysis and SERP Preview features now initialize and fetch page data correctly.