feat: comprehensive i18n coverage — all static text + auto-translate toasts

Locale files expanded to 300+ keys covering all sections:
- Header menu, sidebar, search, viewer, editor, AI toolbar
- Config modal (all sections), help modal, dashboard, graph, mermaid
- Context menus, offline UI, palette, auth, shares, backups
- All toasts and confirmations

Automatic data-i18n tagging in index.html (177 attributes)
via Python script — handles all user-facing static text nodes.
Zero concatenation errors verified.

i18n.js: added tr() reverse-lookup function
(French text → key → current locale translation)

ui.js: showToast() now auto-translates via tr()
— all 24 existing toast call sites benefit automatically

frontend/js/i18n.js

@@ -51,6 +51,30 @@ export function t(key, params) {
   return s;
 }
 
+// ── Reverse-lookup cache (French text → key) ────────────────────────────
+let _reverseCache = null;
+
+function _buildReverseCache() {
+  _reverseCache = {};
+  for (const [k, v] of Object.entries(_fallbackStrings)) {
+    if (k.startsWith('_')) continue;
+    // Prefer shorter keys when multiple keys map to the same French text
+    if (!_reverseCache[v] || k.length < _reverseCache[v].length) {
+      _reverseCache[v] = k;
+    }
+  }
+}
+
+/** Translate a French string to the current locale by reverse lookup.
+ *  "Paramètres client sauvegardés" → fr: same, en: "Client settings saved".
+ *  If the string is not found in the FR locale, returns it unchanged. */
+export function tr(frenchText) {
+  if (!frenchText || _currentLocale === 'fr') return frenchText;
+  if (!_reverseCache) _buildReverseCache();
+  var key = _reverseCache[frenchText];
+  return key ? t(key) : frenchText;
+}
+
 /** Change locale and re-translate the entire DOM.
  *  Persists to localStorage. */
 export async function setLocale(locale) {