Back-to-top button in the status bar

A small arrow button at the right end of the status bar, shown once the
page has scrolled 400 px, scrolls smoothly back to the top (instantly
when the system asks for reduced motion). Living in the fixed status bar,
it never covers a log row or the details panel, on desktop and phones.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

web/index.html

@@ -115,6 +115,9 @@
   <footer class="status">
     <span id="conn" class="conn"></span>
     <span id="stats" class="muted"></span>
+    <button id="toTop" class="icon-btn to-top" type="button" data-i18n-title="toTop" data-i18n-aria="toTop" hidden>
+      <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M12 19V5M5 12l7-7 7 7"/></svg>
+    </button>
   </footer>
 
   <dialog id="settingsDlg" class="settings" aria-labelledby="settingsTitle">