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/backup-manager.js

@@ -235,7 +235,7 @@ async function previewBackup(fpath) {
 async function restoreBackup(vault, file) {
   // Find the backup timestamp from the currently selected backup
   var item = document.querySelector('.bm-backup-item.active');
-  if (!item) { showToast('Selectionnez un backup a restaurer', 'error'); return; }
+  if (!item) { showToast(t('backup.select_to_restore'), 'error'); return; }
 
   var backupPath = item.dataset.path;
   // Extract timestamp from the filename
@@ -248,7 +248,7 @@ async function restoreBackup(vault, file) {
       break;
     }
   }
-  if (!ts) { showToast('Impossible de trouver le timestamp du backup', 'error'); return; }
+  if (!ts) { showToast(t('backup.timestamp_not_found'), 'error'); return; }
 
   if (!confirm('Restaurer "' + file + '" a la version du backup selectionne ?\nLa version actuelle sera sauvegardee.')) return;
 
@@ -259,7 +259,7 @@ async function restoreBackup(vault, file) {
       body: JSON.stringify({ version: ts }),
     });
     if (resp && resp.success) {
-      showToast('Fichier restaure avec succes', 'success');
+      showToast(t('backup.file_restored'), 'success');
       load();
     }
   } catch (err) {