i18n: fix hidden files save button — hardcoded French in JS

- Button label: sidebar.tab_backups -> config.save
- JS: Sauvegarde... -> t('config.saving')
- JS: Sauvegarder -> t('config.save')
- JS: Erreur inconnue -> t('config.error_unknown')
- JS: Erreur: -> t('config.error_prefix')
- +3 new locale keys (1261 total)

frontend/js/config.js

@@ -1038,7 +1038,7 @@ async function saveHiddenFilesSettings() {
   const btn = document.getElementById("cfg-save-hidden-files");
   if (btn) {
     btn.disabled = true;
-    btn.textContent = "Sauvegarde...";
+    btn.textContent = t("config.saving") + "...";
   }
 
   try {
@@ -1073,12 +1073,12 @@ async function saveHiddenFilesSettings() {
     await Promise.all([refreshSidebarTreePreservingState(), refreshTagsForContext()]);
   } catch (err) {
     console.error("Failed to save hidden files settings:", err);
-    const errorMsg = err.message || "Erreur inconnue";
-    showToast(`Erreur: ${errorMsg}`, "error");
+    const errorMsg = err.message || t("config.error_unknown");
+    showToast(t("config.error_prefix") + ": " + errorMsg, "error");
   } finally {
     if (btn) {
       btn.disabled = false;
-      btn.textContent = "💾 Sauvegarder";
+      btn.textContent = "💾 " + t("config.save");
     }
   }
 }