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/series/page.tsx

@@ -1,4 +1,5 @@
 import { fetchAllSeries, fetchLibraries, fetchSeriesStatuses, LibraryDto, SeriesDto, SeriesPageDto, getBookCoverUrl } from "../../lib/api";
+import { getServerTranslations } from "../../lib/i18n/server";
 import { MarkSeriesReadButton } from "../components/MarkSeriesReadButton";
 import { LiveSearchForm } from "../components/LiveSearchForm";
 import { Card, CardContent, OffsetPagination } from "../components/ui";
@@ -12,6 +13,7 @@ export default async function SeriesPage({
 }: {
   searchParams: Promise<{ [key: string]: string | string[] | undefined }>;
 }) {
+  const { t } = await getServerTranslations();
   const searchParamsAwaited = await searchParams;
   const libraryId = typeof searchParamsAwaited.library === "string" ? searchParamsAwaited.library : undefined;
   const searchQuery = typeof searchParamsAwaited.q === "string" ? searchParamsAwaited.q : "";
@@ -33,39 +35,39 @@ export default async function SeriesPage({
   const series = seriesPage.items;
   const totalPages = Math.ceil(seriesPage.total / limit);
   const sortOptions = [
-    { value: "", label: "Titre" },
-    { value: "latest", label: "Ajout récent" },
+    { value: "", label: t("books.sortTitle") },
+    { value: "latest", label: t("books.sortLatest") },
   ];
 
   const hasFilters = searchQuery || libraryId || readingStatus || sort || seriesStatus || hasMissing;
 
   const libraryOptions = [
-    { value: "", label: "Toutes les bibliothèques" },
+    { value: "", label: t("books.allLibraries") },
     ...libraries.map((lib) => ({ value: lib.id, label: lib.name })),
   ];
 
   const statusOptions = [
-    { value: "", label: "Tous" },
-    { value: "unread", label: "Non lu" },
-    { value: "reading", label: "En cours" },
-    { value: "read", label: "Lu" },
+    { value: "", label: t("common.all") },
+    { value: "unread", label: t("status.unread") },
+    { value: "reading", label: t("status.reading") },
+    { value: "read", label: t("status.read") },
   ];
 
   const KNOWN_STATUSES: Record<string, string> = {
-    ongoing: "En cours",
-    ended: "Terminée",
-    hiatus: "Hiatus",
-    cancelled: "Annulée",
-    upcoming: "À paraître",
+    ongoing: t("seriesStatus.ongoing"),
+    ended: t("seriesStatus.ended"),
+    hiatus: t("seriesStatus.hiatus"),
+    cancelled: t("seriesStatus.cancelled"),
+    upcoming: t("seriesStatus.upcoming"),
   };
   const seriesStatusOptions = [
-    { value: "", label: "Tous les statuts" },
+    { value: "", label: t("seriesStatus.allStatuses") },
     ...dbStatuses.map((s) => ({ value: s, label: KNOWN_STATUSES[s] || s })),
   ];
 
   const missingOptions = [
-    { value: "", label: "Tous" },
-    { value: "true", label: "Livres manquants" },
+    { value: "", label: t("common.all") },
+    { value: "true", label: t("series.missingBooks") },
   ];
 
   return (
@@ -75,7 +77,7 @@ export default async function SeriesPage({
           <svg className="w-8 h-8 text-warning" fill="none" stroke="currentColor" viewBox="0 0 24 24">
             <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M19 11H5m14 0a2 2 0 012 2v6a2 2 0 01-2 2H5a2 2 0 01-2-2v-6a2 2 0 012-2m14 0V9a2 2 0 00-2-2M5 11V9a2 2 0 012-2m0 0V5a2 2 0 012-2h6a2 2 0 012 2v2M7 7h10" />
           </svg>
-          Séries
+          {t("series.title")}
         </h1>
       </div>
 
@@ -84,12 +86,12 @@ export default async function SeriesPage({
           <LiveSearchForm
             basePath="/series"
             fields={[
-              { name: "q", type: "text", label: "Rechercher", placeholder: "Rechercher par nom de série...", className: "flex-1 w-full" },
-              { name: "library", type: "select", label: "Bibliothèque", options: libraryOptions, className: "w-full sm:w-48" },
-              { name: "status", type: "select", label: "Lecture", options: statusOptions, className: "w-full sm:w-36" },
-              { name: "series_status", type: "select", label: "Statut", options: seriesStatusOptions, className: "w-full sm:w-36" },
-              { name: "has_missing", type: "select", label: "Manquant", options: missingOptions, className: "w-full sm:w-36" },
-              { name: "sort", type: "select", label: "Tri", options: sortOptions, className: "w-full sm:w-36" },
+              { name: "q", type: "text", label: t("common.search"), placeholder: t("series.searchPlaceholder"), className: "flex-1 w-full" },
+              { name: "library", type: "select", label: t("books.library"), options: libraryOptions, className: "w-full sm:w-48" },
+              { name: "status", type: "select", label: t("series.reading"), options: statusOptions, className: "w-full sm:w-36" },
+              { name: "series_status", type: "select", label: t("editSeries.status"), options: seriesStatusOptions, className: "w-full sm:w-36" },
+              { name: "has_missing", type: "select", label: t("series.missing"), options: missingOptions, className: "w-full sm:w-36" },
+              { name: "sort", type: "select", label: t("books.sort"), options: sortOptions, className: "w-full sm:w-36" },
             ]}
           />
         </CardContent>
@@ -97,8 +99,8 @@ export default async function SeriesPage({
 
       {/* Results count */}
       <p className="text-sm text-muted-foreground mb-4">
-        {seriesPage.total} séries
-        {searchQuery && <> correspondant à &quot;{searchQuery}&quot;</>}
+        {seriesPage.total} {t("series.title").toLowerCase()}
+        {searchQuery && <> {t("series.matchingQuery")} &quot;{searchQuery}&quot;</>}
       </p>
 
       {/* Series Grid */}
@@ -119,7 +121,7 @@ export default async function SeriesPage({
                   <div className="aspect-[2/3] relative bg-muted/50">
                     <Image
                       src={getBookCoverUrl(s.first_book_id)}
-                      alt={`Couverture de ${s.name}`}
+                      alt={t("books.coverOf", { name: s.name })}
                       fill
                       className="object-cover"
                       unoptimized
@@ -127,11 +129,11 @@ export default async function SeriesPage({
                   </div>
                   <div className="p-3">
                     <h3 className="font-medium text-foreground truncate text-sm" title={s.name}>
-                      {s.name === "unclassified" ? "Non classé" : s.name}
+                      {s.name === "unclassified" ? t("books.unclassified") : s.name}
                     </h3>
                     <div className="flex items-center justify-between mt-1">
                       <p className="text-xs text-muted-foreground">
-                        {s.books_read_count}/{s.book_count} lu{s.book_count !== 1 ? "s" : ""}
+                        {t("series.readCount", { read: String(s.books_read_count), total: String(s.book_count), plural: s.book_count !== 1 ? "s" : "" })}
                       </p>
                       <MarkSeriesReadButton
                         seriesName={s.name}
@@ -148,17 +150,12 @@ export default async function SeriesPage({
                           s.series_status === "cancelled" ? "bg-red-500/15 text-red-600" :
                           "bg-muted text-muted-foreground"
                         }`}>
-                          {s.series_status === "ongoing" ? "En cours" :
-                           s.series_status === "ended" ? "Terminée" :
-                           s.series_status === "hiatus" ? "Hiatus" :
-                           s.series_status === "cancelled" ? "Annulée" :
-                           s.series_status === "upcoming" ? "À paraître" :
-                           s.series_status}
+                          {KNOWN_STATUSES[s.series_status] || s.series_status}
                         </span>
                       )}
                       {s.missing_count != null && s.missing_count > 0 && (
                         <span className="text-[10px] px-1.5 py-0.5 rounded-full font-medium bg-yellow-500/15 text-yellow-600">
-                          {s.missing_count} manquant{s.missing_count > 1 ? "s" : ""}
+                          {t("series.missingCount", { count: String(s.missing_count), plural: s.missing_count > 1 ? "s" : "" })}
                         </span>
                       )}
                     </div>
@@ -183,7 +180,7 @@ export default async function SeriesPage({
             </svg>
           </div>
           <p className="text-muted-foreground text-lg">
-            {hasFilters ? "Aucune série trouvée correspondant à vos filtres" : "Aucune série disponible"}
+            {hasFilters ? t("series.noResults") : t("series.noSeries")}
           </p>
         </div>
       )}