Restore code to featureful condition

Restore src/, upgrade to Next.js 14, refactor Next config
- Restore src/ to include all subdirectories for the application
  - Bring src/ in line with changes PR #56 https://github.com/codeforbtv/baby-equipment-exchange/pull/56 and BlueMycon:manage-dependencies branch.
    - Upgrade and remove dependencies.
    - Include the refactor of the Login in src/app/login/page.tsx.
  - Ensure react-burger-menu dependency is removed, since it is no longer being used.
- Upgrade to Next 14
  - Ensure our code is compatible with breaking changes as per https://nextjs.org/blog/next-14.
  - Note that Next 15 has just dropped. It has a longer list with more significant breaking changes, so will take a little longer to review and migrate.
- Refactor next.config.js
  - Ensure raw JSON is is read from the Firebase config file and set as value of NEXT_PUBLIC_FIREBASE_CONFIG environment variable for the browser's consumption. Raw JSON is expected by src/api/firebase.ts.
- Restore Cypress for end-to-end testing.
Restore Cloud Functions directory for now
- Restore functions/ directory until we complete any API migrations as we migrate to the Firebase App Hosting platform

@nfloersch @bryanparmelee Could you please review this PR?

codeforbtv / baby-equipment-exchange