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/viewer.js

@@ -360,7 +360,7 @@ export async function openFile(vaultName, filePath) {
         offline: true
       };
       renderFile(fakeData);
-      showToast('Fichier charge depuis le cache hors-ligne', 'info');
+      showToast(t('viewer.loaded_from_cache'), 'info');
     } else {
       area.innerHTML = '<div class="welcome"><p style="color:var(--text-muted)">Impossible de charger le fichier — vous etes hors-ligne et ce fichier n\'est pas en cache.</p></div>';
     }
@@ -488,11 +488,11 @@ export function renderFile(data) {
         document.execCommand("copy");
         document.body.removeChild(ta);
       }
-      copyBtn.lastChild.textContent = "Copié !";
-      setTimeout(() => (copyBtn.lastChild.textContent = "Copier"), 1500);
+      copyBtn.lastChild.textContent = t('viewer.copied');
+      setTimeout(() => (copyBtn.lastChild.textContent = t('viewer.copy')), 1500);
     } catch (err) {
       console.error("Copy error:", err);
-      showToast("Erreur lors de la copie", "error");
+      showToast(t("viewer.copy_error"), "error");
     }
   });
 
@@ -567,7 +567,7 @@ export function renderFile(data) {
       const shares = await api("/api/shares");
       if (shares.some(s => s.vault === data.vault && s.path === data.path)) {
         shareBtn.classList.add("shared");
-        shareBtn.title = "Document partagé — cliquer pour gérer";
+        shareBtn.title = t('viewer.shared_doc_title');
       }
     } catch (e) { /* ignore */ }
   })();
@@ -1331,7 +1331,7 @@ 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) {