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/ui.js

@@ -153,7 +153,7 @@ export function applyTheme(theme) {
     if (icon) {
       icon.setAttribute("data-lucide", theme === "dark" ? "moon" : "sun");
     }
-    themeLabel.textContent = theme === "dark" ? "Sombre" : "Clair";
+    themeLabel.textContent = theme === "dark" ? t('theme.dark') : t('theme.light');
     safeCreateIcons();
   }
 
@@ -860,7 +860,7 @@ const FileOperations = {
     // Find the text span (.tree-item-text) — tree items use spans, not direct text nodes
     const textSpan = item.querySelector('.tree-item-text');
     if (!textSpan) {
-      showToast('Impossible de renommer cet élément', 'error');
+      showToast(t('ui.cannot_rename'), 'error');
       return;
     }
 
@@ -1586,7 +1586,7 @@ async function init() {
       }
     } catch (err) {
       console.error("Failed to initialize ObsiGate:", err);
-      showToast("Erreur lors de l'initialisation", "error");
+      showToast(t("error.initialization"), "error");
     }
   }