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/app.js

@@ -126,6 +126,7 @@ const I18N = {
     hDivision: (s) => `interval ${s}`, hCapped: 'enlarged',
     hCappedTitle: 'The chosen division would exceed 300 intervals over this range: it has been enlarged.',
     hUnzoom: '× Reset zoom', unitDay: 'd',
+    toTop: 'Back to top',
     colRcv: 'Received', colMt: 'Message time', colSev: 'Severity', colHost: 'Host', colApp: 'App', colMsg: 'Message',
     colGrip: 'Drag to resize, double-click for the automatic width',
     resetCols: 'Reset column widths', colsReset: 'Column widths reset',
@@ -254,6 +255,7 @@ const I18N = {
     hDivision: (s) => `intervalle ${s}`, hCapped: 'élargi',
     hCappedTitle: 'La division choisie dépasserait 300 intervalles sur cette plage : elle a été élargie.',
     hUnzoom: '× Annuler le zoom', unitDay: 'j',
+    toTop: 'Revenir en haut',
     colRcv: 'Réception', colMt: 'Heure message', colSev: 'Sévérité', colHost: 'Hôte', colApp: 'App', colMsg: 'Message',
     colGrip: 'Glisser pour redimensionner, double-clic pour la largeur automatique',
     resetCols: 'Réinitialiser les colonnes', colsReset: 'Largeurs de colonnes réinitialisées',
@@ -1517,6 +1519,15 @@ $('#newPill').addEventListener('click', () => {
 });
 window.addEventListener('scroll', debounce(() => { if (window.scrollY <= 80) flushPending(); }, 150), { passive: true });
 
+// "Back to top" button, shown once the page has scrolled a little.
+const syncToTop = () => { $('#toTop').hidden = window.scrollY < 400; };
+window.addEventListener('scroll', syncToTop, { passive: true });
+$('#toTop').addEventListener('click', () => {
+  const smooth = !matchMedia('(prefers-reduced-motion: reduce)').matches;
+  window.scrollTo({ top: 0, behavior: smooth ? 'smooth' : 'auto' });
+});
+syncToTop();
+
 $('#live').addEventListener('click', () => {
   state.live = !state.live;
   store.set('live', state.live ? '1' : '0');