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/dictionaries.ts

@@ -0,0 +1,35 @@
+import type { Locale } from "./types";
+import type { TranslationKey } from "./fr";
+
+const dictionaries: Record<Locale, () => Promise<Record<TranslationKey, string>>> = {
+  fr: () => import("./fr").then((m) => m.default),
+  en: () => import("./en").then((m) => m.default),
+};
+
+export async function getDictionary(locale: Locale): Promise<Record<TranslationKey, string>> {
+  return dictionaries[locale]();
+}
+
+// Synchronous versions for client-side use
+import fr from "./fr";
+import en from "./en";
+
+const dictionariesSync: Record<Locale, Record<TranslationKey, string>> = { fr, en };
+
+export function getDictionarySync(locale: Locale): Record<TranslationKey, string> {
+  return dictionariesSync[locale];
+}
+
+export type TranslateFunction = (key: TranslationKey, params?: Record<string, string | number>) => string;
+
+export function createTranslateFunction(dict: Record<TranslationKey, string>): TranslateFunction {
+  return (key: TranslationKey, params?: Record<string, string | number>): string => {
+    let value: string = dict[key] ?? key;
+    if (params) {
+      for (const [k, v] of Object.entries(params)) {
+        value = value.replace(new RegExp(`\\{\\{${k}\\}\\}`, "g"), String(v));
+      }
+    }
+    return value;
+  };
+}