feat(diagnostic): Knopf nach oben — schwebt unten rechts beim Scrollen

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

diagnostic/index.html

@@ -10870,5 +10870,24 @@
       <div id="versions-body" style="overflow-y:auto;padding:4px 12px;"></div>
     </div>
   </div>
+
+  <!-- Nach oben: schwebt unten rechts, sobald man ein Stueck gescrollt hat -->
+  <button id="to-top" type="button" title="Nach oben" aria-label="Nach oben"
+          onclick="window.scrollTo({ top: 0, behavior: 'smooth' })"
+          style="position:fixed;right:20px;bottom:20px;z-index:900;width:42px;height:42px;border-radius:50%;
+                 border:1px solid #2A2A3E;background:#1E1E2E;color:#E0E0F0;font-size:20px;line-height:1;
+                 cursor:pointer;box-shadow:0 2px 8px rgba(0,0,0,.5);opacity:0;pointer-events:none;transition:opacity .2s;">↑</button>
+  <script>
+    (function () {
+      const b = document.getElementById('to-top');
+      const upd = () => {
+        const on = window.scrollY > 400;
+        b.style.opacity = on ? '0.85' : '0';
+        b.style.pointerEvents = on ? 'auto' : 'none';
+      };
+      window.addEventListener('scroll', upd, { passive: true });
+      upd();
+    })();
+  </script>
 </body>
 </html>