Add setTimeout(0) to allow spinner to render before heavy computation

The issue was that the synchronous operations were executing immediately after showing the spinner, without giving the browser a chance to paint the DOM changes. By adding `await new Promise(resolve => setTimeout(resolve, 0))`, we yield control back to the event loop, allowing the browser to render the spinner before the expensive cluster sorting operations begin.

This ensures the spinner is visible during the 1-3 second delay on large clusters.

Co-authored-by: lstein <111189+lstein@users.noreply.github.com>

Author: Copilot

photomap/frontend/static/javascript/umap.js

@@ -1260,6 +1260,9 @@ async function handleClusterClick(clickedIndex) {
   // Show spinner immediately to provide visual feedback
   showUmapSpinner();
 
+  // Yield to the browser to allow spinner to render before heavy computation
+  await new Promise(resolve => setTimeout(resolve, 0));
+  
   try {
     const clickedCluster = clickedPoint.cluster;
     const clusterColor = getClusterColor(clickedCluster);
@@ -1302,6 +1305,9 @@ async function handleImageClick(clickedIndex) {
   // Show spinner immediately to provide visual feedback
   showUmapSpinner();
 
+  // Yield to the browser to allow spinner to render before heavy computation
+  await new Promise(resolve => setTimeout(resolve, 0));
+  
   try {
     // Clear any existing search selection
     exitSearchMode();