fix: resolve nested i18n keys via dot notation in t()

The locale JSON files use nested structure (e.g. {"nav":{"tasks":"…"}}),
but t() did a flat lookup, always falling back to the raw key string.

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

public/i18n.js

@@ -64,9 +64,14 @@ export async function setLocale(locale) {
   window.dispatchEvent(new CustomEvent('locale-changed', { detail: { locale } }));
 }
 
+/** Hilfsfunktion: Dot-Notation in verschachteltem Objekt auflösen */
+function resolve(obj, key) {
+  return key.split('.').reduce((o, k) => (o != null ? o[k] : undefined), obj);
+}
+
 /** Übersetzungsfunktion mit Platzhalter-Unterstützung {{variable}} */
 export function t(key, params = {}) {
-  let str = translations[key] ?? fallbackTranslations[key] ?? key;
+  let str = resolve(translations, key) ?? resolve(fallbackTranslations, key) ?? key;
   for (const [k, v] of Object.entries(params)) {
     str = str.replaceAll(`{{${k}}}`, String(v));
   }