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/auth.js

@@ -195,7 +195,7 @@ const AuthManager = {
       userMenu.innerHTML = "";
       return;
     }
-    userMenu.innerHTML = '<span class="user-display-name">' + (user.display_name || user.username) + "</span>" + '<button class="btn-logout" id="logout-btn" title="Déconnexion"><i data-lucide="log-out" style="width:14px;height:14px"></i></button>';
+    userMenu.innerHTML = '<span class="user-display-name">' + (user.display_name || user.username) + "</span>" + '<button class="btn-logout" id="logout-btn" title="' + t('auth.logout_title') + '"><i data-lucide="log-out" style="width:14px;height:14px"></i></button>';
     safeCreateIcons();
 
     const logoutBtn = document.getElementById("logout-btn");
@@ -535,14 +535,14 @@ const AdminPanel = {
   async _deleteUser(username) {
     const currentUser = AuthManager.getUser();
     if (currentUser && currentUser.username === username) {
-      showToast("Impossible de supprimer son propre compte", "error");
+      showToast(t("auth.cannot_delete_self"), "error");
       return;
     }
     if (!confirm("Supprimer l'utilisateur \"" + username + '" ?')) return;
     try {
       await api("/api/auth/admin/users/" + username, { method: "DELETE" });
       this._loadUsers();
-      showToast("Utilisateur supprimé", "success");
+      showToast(t("auth.user_deleted"), "success");
     } catch (err) {
       showToast(err.message, "error");
     }