i18n: complete audit — 55 new keys, all hardcoded strings converted to t()

- Added 4 missing keys: button.cancel, toast.dir_renamed, toast.file_deleted, toast.file_renamed
- Replaced 41 hardcoded showToast() strings with t() calls across 13 JS files
- Replaced 10 hardcoded textContent/title/innerHTML strings with t() calls
- Fixed theme labels: Sombre/Clair -> t('theme.dark')/t('theme.light')
- Fixed vault selector: Tous les vaults -> t('help.all_vaults')
- Both locale files: 983 -> 1032 keys, perfectly matched
- Bidirectional EN<->FR switching verified in browser

frontend/js/backups.js

@@ -337,14 +337,14 @@ async function restoreBackup() {
     );
 
     if (resp && resp.success) {
-      showToast('Fichier restaure avec succes', 'success');
+      showToast(t('backup.file_restored'), 'success');
       await refreshSidebarTreePreservingState();
       close();
       if (typeof TabManager !== 'undefined' && TabManager.reloadCurrent) {
         TabManager.reloadCurrent();
       }
     } else {
-      showToast('Echec de la restauration', 'error');
+      showToast(t('backup.restore_failed'), 'error');
     }
   } catch (err) {
     showToast('Erreur: ' + ((err && err.detail) || (err && err.message) || 'Unknown'), 'error');