`useBaseUrl` breaks Rules of Hooks in a few components

[Anshumancanrock]

useBaseUrl is a React hook, but a few spots call it inside conditionals, .map(), or after an early return:

• src/pages/index.js : inside ecosystem / vendor maps
• src/pages/case-studies.js : inside caseStudies.map
• src/theme/BlogPostItem/Header/index.js : only when cover is set
• src/theme/Footer/index.js : useEffect / useBaseUrl after if (!footer) return null

No ESLint in the repo (npm run lint is markdown only), so nothing catches it. Would be good to fix the call sites and add eslint-plugin-react-hooks.