feat: Scroll-to-Top Button (erscheint ab 300px Scroll)

frontend/src/App.jsx

@@ -3215,6 +3215,51 @@ function PublicUpload({ token }) {
   );
 }
 
+// ── Scroll-to-Top Button ──────────────────────────────────────────────────
+function ScrollToTop({ scrollRef }) {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    const el = scrollRef?.current;
+    if (!el) return;
+    const onScroll = () => setVisible(el.scrollTop > 300);
+    el.addEventListener('scroll', onScroll, { passive: true });
+    return () => el.removeEventListener('scroll', onScroll);
+  }, [scrollRef]);
+
+  if (!visible) return null;
+
+  return (
+    <button
+      onClick={() => scrollRef.current?.scrollTo({ top: 0, behavior: 'smooth' })}
+      style={{
+        position: 'fixed',
+        bottom: 'calc(70px + env(safe-area-inset-bottom, 0px))',
+        right: 20,
+        zIndex: 900,
+        width: 40,
+        height: 40,
+        borderRadius: '50%',
+        background: 'rgba(255,255,255,0.08)',
+        border: '1px solid rgba(255,255,255,0.15)',
+        backdropFilter: 'blur(8px)',
+        color: 'rgba(255,255,255,0.7)',
+        fontSize: 16,
+        cursor: 'pointer',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+        boxShadow: '0 4px 16px rgba(0,0,0,0.4)',
+        transition: 'opacity 0.2s',
+      }}
+      title="Nach oben"
+    >
+      ↑
+    </button>
+  );
+}
+
+
 export default function App() {
   // Öffentliche Upload-Seite – kein Login, kein Auth
   const _uploadMatch = window.location.pathname.match(/^\/u\/(.+)$/);
@@ -3303,6 +3348,7 @@ 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';
@@ -3331,7 +3377,7 @@ export default function App() {
            
             unreadMsgs={unreadMsgs}/>
         )}
-        <main style={mainStyle}>
+        <main ref={mainRef} style={mainStyle}>
           {active==='dashboard' && <Dashboard toast={toast} mobile={mobile} setActive={setActive} setDevToolsNav={setDevToolsNav} unreadMsgs={unreadMsgs} unreadBoard={unreadBoard} unreadPromoted={unreadPromoted} onBoardRead={()=>{ setUnreadBoard(0); setUnreadPromoted(0); }} unreadChangelog={unreadChangelog} onChangelogRead={()=>setUnreadChangelog(0)} user={user}/>}
           {active==='admin' && <AdminPanel toast={toast} mobile={mobile} user={user} nav={devToolsNav}/>}
           {activeTool && <activeTool.component toast={toast} mobile={mobile} setActive={setActive} {...(activeTool.id==='devtools'?{nav:devToolsNav}:{})}/>}
@@ -3343,6 +3389,7 @@ export default function App() {
          
           unreadMsgs={unreadMsgs}/>
       )}
+      <ScrollToTop scrollRef={mainRef}/>
       <Toast msg={toastMsg.msg} type={toastMsg.type}/>
     </>
   );