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/sidebar.js

@@ -223,7 +223,7 @@ async function loadVaults() {
   container.innerHTML = "";
 
   // Prepare dropdown options
-  const dropdownOptions = [{ value: "all", text: "Tous les vaults" }, ...vaults.map((v) => ({ value: v.name, text: v.name }))];
+  const dropdownOptions = [{ value: "all", text: t('help.all_vaults') }, ...vaults.map((v) => ({ value: v.name, text: v.name }))];
 
   // Populate custom dropdowns
   populateCustomDropdown("vault-filter-dropdown", dropdownOptions, "all");