feat: rewrite to Next.js v13 app directory

Improvements:
- Hide switch theme input (ugly little white square)
- i18n without subpath (e.g: /fr or /en), same url whatever the locale used

components/Header/SwitchTheme.tsx

@@ -71,7 +71,7 @@ export const SwitchTheme: React.FC = () => {
           data-cy='switch-theme-input'
           type='checkbox'
           aria-label='Dark mode toggle'
-          className='absolute m-[-1px] h-[1px] w-[1px] overflow-hidden border-0 p-0'
+          className='absolute m-[-1px] h-[1px] w-[1px] overflow-hidden border-0 p-0 hidden'
           defaultChecked
         />
       </div>