feat(frontend): scaffold i18n init, fr.json (~140 keys), status map, format helpers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/i18n/index.ts

@@ -0,0 +1,13 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import fr from './fr.json';
+
+i18n.use(initReactI18next).init({
+  resources: { fr: { translation: fr } },
+  lng: 'fr',
+  fallbackLng: 'fr',
+  interpolation: { escapeValue: false },
+  returnEmptyString: false,
+});
+
+export default i18n;