Fix 30s auto-refresh flicker in frontend

Skip the full-page loading state on background refreshes (only show it
on first load or view switch). Replace per-column requestAnimationFrame
appends with a single DocumentFragment swap so the browser never paints
a blank container between clear and insert.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/app.js

@@ -35,6 +35,7 @@ let currentView      = 'restaurants';
 let refreshTimer     = null;
 let zabbixUrl        = '';
 let customerTagValue = 'QWE';
+let hasRendered      = false;
 const REFRESH_MS     = 30_000;
 
 /* ── Boot ────────────────────────────────────────────────────────────────── */
@@ -56,6 +57,7 @@ function initControls() {
       document.querySelectorAll('.toggle-btn').forEach(b => b.classList.remove('active'));
       btn.classList.add('active');
       currentView = btn.dataset.view;
+      hasRendered = false;
       loadData();
     });
   });
@@ -77,7 +79,7 @@ function initControls() {
 /* ── Data loading ────────────────────────────────────────────────────────── */
 async function loadData() {
   setRefreshSpinner(true);
-  showPageLoading();
+  if (!hasRendered) showPageLoading();
 
   try {
     const endpoint = currentView === 'restaurants' ? '/api/restaurants' : '/api/devices';
@@ -88,6 +90,7 @@ async function loadData() {
     renderCountryColumns(items, stats);
     updateSummary(items);
     setLastUpdated(new Date());
+    hasRendered = true;
   } catch (e) {
     showPageError(e.message);
   } finally {
@@ -102,7 +105,6 @@ function scheduleRefresh() {
 
 /* ── Country column layout ───────────────────────────────────────────────── */
 function renderCountryColumns(items, stats = {}) {
-  // Group by country
   const byCountry = {};
   for (const item of items) {
     const key = item.country || '';
@@ -112,20 +114,20 @@ function renderCountryColumns(items, stats = {}) {
 
   const countries = Object.keys(byCountry).sort();
   const wrap = document.getElementById('countries-wrap');
-  wrap.innerHTML = '';
 
   if (countries.length === 0) {
     wrap.innerHTML = '<div class="page-center"><p>No items found.</p></div>';
     return;
   }
 
+  const frag = document.createDocumentFragment();
+  const refs = [];
+
   countries.forEach(country => {
     const col = document.createElement('div');
     col.className = 'country-col';
 
     const colItems = byCountry[country];
-    const problems  = colItems.filter(i => i.severity >= 2).length;
-
     const flagUrl  = countryFlagUrl(country);
     const flagImg  = flagUrl
       ? `<img src="${flagUrl}" alt="" style="height:22px;width:auto;border-radius:2px;flex-shrink:0;">`
@@ -151,13 +153,20 @@ function renderCountryColumns(items, stats = {}) {
     body.appendChild(grid);
     col.appendChild(hdr);
     col.appendChild(body);
-    wrap.appendChild(col);
+    frag.appendChild(col);
 
-    // Compute perRow after layout is in DOM
+    refs.push({ grid, body, colItems });
+  });
+
+  // Single atomic swap — old content replaced in one operation
+  wrap.replaceChildren(frag);
+
+  // Measure layout and render hex items after browser has laid out the new columns
   requestAnimationFrame(() => {
     const hexW   = parseInt(getComputedStyle(document.documentElement).getPropertyValue('--hex-w'));
     const hexGap = parseInt(getComputedStyle(document.documentElement).getPropertyValue('--hex-gap'));
-      const colW   = body.clientWidth - 40; // subtract padding
+    refs.forEach(({ grid, body, colItems }) => {
+      const colW   = body.clientWidth - 40;
       const perRow = Math.max(2, Math.floor(colW / (hexW + hexGap)));
       renderHexGrid(grid, colItems, perRow);
       renderProblemList(body, colItems);