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/app/components/BookPreview.tsx

@@ -2,10 +2,12 @@
 
 import { useState } from "react";
 import Image from "next/image";
+import { useTranslation } from "../../lib/i18n/context";
 
 const PAGE_SIZE = 5;
 
 export function BookPreview({ bookId, pageCount }: { bookId: string; pageCount: number }) {
+  const { t } = useTranslation();
   const [offset, setOffset] = useState(0);
 
   const pages = Array.from({ length: PAGE_SIZE }, (_, i) => offset + i + 1).filter(
@@ -16,9 +18,9 @@ export function BookPreview({ bookId, pageCount }: { bookId: string; pageCount:
     <div className="bg-card rounded-xl border border-border p-6">
       <div className="flex items-center justify-between mb-4">
         <h2 className="text-lg font-semibold text-foreground">
-          Aperçu
+          {t("bookPreview.preview")}
           <span className="ml-2 text-sm font-normal text-muted-foreground">
-            pages {offset + 1}–{Math.min(offset + PAGE_SIZE, pageCount)} / {pageCount}
+            {t("bookPreview.pages", { start: offset + 1, end: Math.min(offset + PAGE_SIZE, pageCount), total: pageCount })}
           </span>
         </h2>
         <div className="flex gap-2">
@@ -27,14 +29,14 @@ export function BookPreview({ bookId, pageCount }: { bookId: string; pageCount:
             disabled={offset === 0}
             className="px-3 py-1.5 text-sm rounded-lg border border-border bg-muted/50 text-foreground hover:bg-muted disabled:opacity-40 disabled:cursor-not-allowed transition-colors"
           >
-            ← Préc.
+            {t("bookPreview.prev")}
           </button>
           <button
             onClick={() => setOffset((o) => Math.min(o + PAGE_SIZE, pageCount - 1))}
             disabled={offset + PAGE_SIZE >= pageCount}
             className="px-3 py-1.5 text-sm rounded-lg border border-border bg-muted/50 text-foreground hover:bg-muted disabled:opacity-40 disabled:cursor-not-allowed transition-colors"
           >
-            Suiv. →
+            {t("bookPreview.next")}
           </button>
         </div>
       </div>