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/server.ts

@@ -0,0 +1,20 @@
+import { cookies } from "next/headers";
+import type { Locale } from "./types";
+import { DEFAULT_LOCALE, LOCALE_COOKIE, LOCALES } from "./types";
+import { getDictionarySync, createTranslateFunction } from "./dictionaries";
+import type { TranslateFunction } from "./dictionaries";
+
+export async function getServerLocale(): Promise<Locale> {
+  const cookieStore = await cookies();
+  const raw = cookieStore.get(LOCALE_COOKIE)?.value;
+  if (raw && LOCALES.includes(raw as Locale)) {
+    return raw as Locale;
+  }
+  return DEFAULT_LOCALE;
+}
+
+export async function getServerTranslations(): Promise<{ t: TranslateFunction; locale: Locale }> {
+  const locale = await getServerLocale();
+  const dict = getDictionarySync(locale);
+  return { t: createTranslateFunction(dict), locale };
+}