fix(i18n): a locale edit hot-swaps the texts instead of re-running main.tsx

The locale JSON's importer did not accept HMR, so the update climbed to
main.tsx, whose re-run called createRoot on the live container again; the two
roots tore each other's DOM down ('removeChild ... not a child' by the dozen),
each a fatal toast evicting an old notification. Dev only.

frontend/src/i18n/index.ts

@@ -40,15 +40,26 @@ function detectInitialLang(): LangCode {
   return isSupported(nav) ? nav : "en";
 }
 
-const initial = detectInitialLang();
-void i18n.use(initReactI18next).init({
-  resources,
-  lng: initial,
-  fallbackLng: "en",
-  interpolation: { escapeValue: false },
-  returnNull: false,
-});
-document.documentElement.lang = initial;
+// Dev only: a locale edit re-runs THIS module (it accepts its own update, below) instead of climbing to
+// main.tsx — a re-run main.tsx called createRoot on the live container a second time, and the two roots
+// tore each other's DOM down ("removeChild … not a child" by the dozen, each one a fatal toast that evicted
+// an old notification). On a re-run the i18next singleton is already up: swap the texts in and re-render.
+if (i18n.isInitialized) {
+  for (const [lng, r] of Object.entries(resources))
+    i18n.addResourceBundle(lng, "translation", r.translation, true, true);
+  void i18n.changeLanguage(i18n.language);
+} else {
+  const initial = detectInitialLang();
+  void i18n.use(initReactI18next).init({
+    resources,
+    lng: initial,
+    fallbackLng: "en",
+    interpolation: { escapeValue: false },
+    returnNull: false,
+  });
+  document.documentElement.lang = initial;
+}
+if (import.meta.hot) import.meta.hot.accept();
 
 export function setLanguage(code: LangCode): void {
   void i18n.changeLanguage(code);