Docs: Friday Cleanup: docs across 2 areas

[moxie-docs]

Covers these changes:

• Refresh quiet sprint with focus on cleanup and docs readiness
• Add docs for Document environment variables used by the frontend and how they reach the code

Documents how the CRA-based frontend is built and started, and clarifies how environment variables are recognized by react-scripts and reach the code. This helps maintainers quickly run the app, understand the entry point behavior, and correctly wire environment variables during development and builds.

What these docs cover

• Entry point frontend/src/index.js creates a React 18 root, renders inside React.StrictMode, and calls reportWebVitals(), with no environment variable usage in this file
• frontend/package.json drives execution via react-scripts 5.0.1 with scripts start, build, test, eject, and defines React 18 dependencies, ESLint presets (react-app, react-app/jest), and Browserslist targets (production >0.2%, not dead, not op_mini all; development last 1 version of Chrome/Firefox/Safari)
• Under CRA, only REACT_APP_ prefixed variables are exposed to the client; variables are read from shell and .env* files at start/build and inlined at build time
• NODE_ENV is managed by react-scripts (development for start, production for build) and should not be manually overridden
• Environment variable changes require restarting the dev server or rebuilding, since values are captured at dev-server startup/build time
• README reflects standard CRA guidance for start/test/build/eject and aligns with the scripts in package.json

How to review

• Verify frontend/package.json lists react-scripts version 5.0.1 and scripts mapping exactly to react-scripts start, build, test, eject
• Check frontend/src/index.js uses ReactDOM.createRoot(document.getElementById('root')) and renders <React.StrictMode></React.StrictMode>
• Confirm frontend/src/index.js imports reportWebVitals from './reportWebVitals' and invokes reportWebVitals() with no arguments
• Ensure frontend/package.json Browserslist matches production >0.2%, not dead, not op_mini all and development last 1 Chrome/Firefox/Safari versions

Documentation updated

• moxie-docs/frontend/frontend-package.md
• moxie-docs/frontend/frontend.md

Referenced sources

• frontend/package.json
• frontend/src/index.js
• frontend/README.md
• moxie-docs/frontend/build-and-env.md
• frontend/package.json
• frontend/src/index.js
• frontend/README.md