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

utils/constants.ts

@@ -0,0 +1,8 @@
+/** How long in milliseconds, until the cookie expires (10 years). */
+export const COOKIE_MAX_AGE = 10 * 365.25 * 24 * 60 * 60 * 1000
+
+export const AVAILABLE_LOCALES = ['en-US', 'fr-FR'] as const
+
+export type AvailableLocale = (typeof AVAILABLE_LOCALES)[number]
+
+export const DEFAULT_LOCALE = 'en-US' satisfies AvailableLocale