Add preloader and disable service worker to prevent blank screen issues

Introduces a visual preloader to the client's index.html and disables the service worker in main.tsx. Additionally, server-side middleware in index.ts is added to set no-cache headers for HTML pages, aiming to resolve the reported blank screen problem.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 890577b1-c154-40a4-a177-a0c6d55320c3
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/8cc42625-c1f5-4e43-99bd-77f2c4dedee2/890577b1-c154-40a4-a177-a0c6d55320c3/DyHCx4q

client/src/main.tsx

@@ -4,18 +4,18 @@ import "./index.css";
 
 console.log('🎬 main.tsx starting...');
 
-// Registracija Service Worker-ja za PWA funkcionalnost
-if ('serviceWorker' in navigator) {
-  window.addEventListener('load', () => {
-    navigator.serviceWorker.register('/sw.js')
-      .then((registration) => {
-        console.log('SW registered: ', registration);
-      })
-      .catch((registrationError) => {
-        console.log('SW registration failed: ', registrationError);
-      });
-  });
-}
+// Disable Service Worker for now - can cause caching issues
+// if ('serviceWorker' in navigator) {
+//   window.addEventListener('load', () => {
+//     navigator.serviceWorker.register('/sw.js')
+//       .then((registration) => {
+//         console.log('SW registered: ', registration);
+//       })
+//       .catch((registrationError) => {
+//         console.log('SW registration failed: ', registrationError);
+//       });
+//   });
+// }
 
 try {
   const rootElement = document.getElementById("root");
@@ -32,6 +32,19 @@ try {
   root.render(<App />);
   
   console.log('✅ React app mounted successfully');
+  
+  // Remove preloader once React app has mounted
+  setTimeout(() => {
+    const preloader = document.getElementById('app-preloader');
+    if (preloader) {
+      preloader.style.opacity = '0';
+      preloader.style.transition = 'opacity 0.5s ease-out';
+      setTimeout(() => {
+        preloader.remove();
+        console.log('🎨 Preloader removed');
+      }, 500);
+    }
+  }, 100);
 } catch (error) {
   console.error('❌ Failed to mount React app:', error);