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/sync.js

@@ -187,7 +187,7 @@ export const IndexUpdateManager = (() => {
   async function _onIndexReloaded(data) {
     connectionState = "syncing";
     _updateBadge();
-    showToast("Index complet rechargé", "info");
+    showToast(t("sync.index_reloaded"), "info");
     try {
       await Promise.all([loadVaults(), loadTags()]);
     } catch (err) {
@@ -562,7 +562,7 @@ window.addEventListener("beforeinstallprompt", (e) => {
 // Log when app is installed
 window.addEventListener("appinstalled", () => {
   console.log("[PWA] ObsiGate has been installed");
-  showToast("ObsiGate installé avec succès !", "success");
+  showToast(t("sync.install_success"), "success");
 });
 
 // ── Dashboard tab switching (runs on page load and after rebuild) ──