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/ConvertButton.tsx

@@ -3,6 +3,7 @@
 import { useState } from "react";
 import Link from "next/link";
 import { Button } from "./ui";
+import { useTranslation } from "../../lib/i18n/context";
 
 interface ConvertButtonProps {
   bookId: string;
@@ -15,6 +16,7 @@ type ConvertState =
   | { type: "error"; message: string };
 
 export function ConvertButton({ bookId }: ConvertButtonProps) {
+  const { t } = useTranslation();
   const [state, setState] = useState<ConvertState>({ type: "idle" });
 
   const handleConvert = async () => {
@@ -23,22 +25,22 @@ export function ConvertButton({ bookId }: ConvertButtonProps) {
       const res = await fetch(`/api/books/${bookId}/convert`, { method: "POST" });
       if (!res.ok) {
         const body = await res.json().catch(() => ({ error: res.statusText }));
-        setState({ type: "error", message: body.error || "Échec de la conversion" });
+        setState({ type: "error", message: body.error || t("convert.failed") });
         return;
       }
       const job = await res.json();
       setState({ type: "success", jobId: job.id });
     } catch (err) {
-      setState({ type: "error", message: err instanceof Error ? err.message : "Erreur inconnue" });
+      setState({ type: "error", message: err instanceof Error ? err.message : t("convert.unknownError") });
     }
   };
 
   if (state.type === "success") {
     return (
       <div className="flex items-center gap-2 text-sm text-success">
-        <span>Conversion lancée.</span>
+        <span>{t("convert.started")}</span>
         <Link href={`/jobs/${state.jobId}`} className="text-primary hover:underline font-medium">
-          Voir la tâche →
+          {t("convert.viewJob")}
         </Link>
       </div>
     );
@@ -52,7 +54,7 @@ export function ConvertButton({ bookId }: ConvertButtonProps) {
           className="text-xs text-muted-foreground hover:underline text-left"
           onClick={() => setState({ type: "idle" })}
         >
-          Fermer
+          {t("common.close")}
         </button>
       </div>
     );
@@ -65,7 +67,7 @@ export function ConvertButton({ bookId }: ConvertButtonProps) {
       onClick={handleConvert}
       disabled={state.type === "loading"}
     >
-      {state.type === "loading" ? "Conversion…" : "Convertir en CBZ"}
+      {state.type === "loading" ? t("convert.converting") : t("convert.convertToCbz")}
     </Button>
   );
 }