feat: add i18n support (FR/EN) to backoffice with English as default

Implement full internationalization for the Next.js backoffice:
- i18n infrastructure: type-safe dictionaries (fr.ts/en.ts), cookie-based locale detection, React Context for client components, server-side translation helper
- Language selector in Settings page (General tab) with cookie + DB persistence
- All ~35 pages and components translated via t() / useTranslation()
- Default locale set to English, French available via settings

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/lib/i18n/index.ts

@@ -0,0 +1,5 @@
+export { type Locale, DEFAULT_LOCALE, LOCALE_COOKIE, LOCALES } from "./types";
+export { getDictionary, getDictionarySync, createTranslateFunction, type TranslateFunction } from "./dictionaries";
+export { getServerLocale, getServerTranslations } from "./server";
+export { LocaleProvider, useTranslation } from "./context";
+export { type TranslationKey } from "./fr";