fix: mainRef vor Early Returns (React Hooks Rule)

frontend/src/App.jsx

@@ -3268,6 +3268,7 @@ export default function App() {
   if (_shareMatch) return <PublicFileShare token={_shareMatch[1]}/>;
 
   const mobile = useIsMobile();
+  const mainRef = useRef(null);
   const [splash, setSplash] = useState(() => {
     // Only show splash in standalone PWA mode, and only once per session
     return window.matchMedia('(display-mode: standalone)').matches && !sessionStorage.getItem('splash_shown');
@@ -3348,7 +3349,6 @@ export default function App() {
   if(!user) return <Login onLogin={u=>setUser(u)}/>;
 
   const activeTool = TOOLS.find(t=>t.id===active);
-  const mainRef = useRef(null);
 
   // Mobile padding bottom für fixed nav
   const isNachrichten = active === 'nachrichten';