Use npm as the only package manager. The repository has both `package-lock.json` and `pnpm-lock.yaml`, so versions can differ between developers, and not everyone can run pnpm locally. **Done when:** - [ ] `pnpm-lock.yaml` is removed - [ ] The pnpm entries in `.gitignore` and `.prettierignore` are removed, and `package-lock.json` is ignored by Prettier - [ ] The README uses `npm ci` and `npm run ...` - [ ] `engines` in `package.json` declares the Node version - [ ] `npm ci`, lint, build and tests pass from a clean checkout - [ ] When `backend/` is added (#21), it uses npm workspaces