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/offline.js

@@ -21,9 +21,9 @@ export async function initOffline() {
     updateOfflineIndicator();
     if (online) {
       syncPendingChanges();
-      showToast('Connexion retablie — synchronisation en cours...', 'info');
+      showToast(t('offline.reconnected'), 'info');
     } else {
-      showToast('Mode hors-ligne — vos modifications seront synchronisees au retour', 'warning');
+      showToast(t('offline.mode_offline'), 'warning');
     }
   });
 
@@ -182,7 +182,7 @@ export async function syncPendingChanges() {
     updateOfflineIndicator();
 
     if (remaining === 0) {
-      showToast('Toutes les modifications ont ete synchronisees', 'success');
+      showToast(t('offline.all_synced'), 'success');
     } else {
       showToast(remaining + ' modification(s) en attente de resolution', 'warning');
     }