FEAT-13: singular-aware contract count text (1 Vertrag / n Vertraege)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gerbil-manager-web/src/pages/VertraegeListPage.tsx

@@ -58,9 +58,7 @@ export default function VertraegeListPage() {
       <header className="page-head">
         <div>
           <h2>{t.title}</h2>
-          <p className="muted">
-            {totalCount} {t.countLabel}
-          </p>
+          <p className="muted">{t.countText(totalCount)}</p>
         </div>
         <div className="head-actions">
           <Link to="/vertraege/neu" className="btn btn--primary">