[code] created app.js and service-worker.js, these files are key for offline and PWA functionality, also updated all project HTML files to reflect these new files and their relevant links.

app.js

@@ -0,0 +1,8 @@
+if ('serviceWorker' in navigator) {         // Setup — initialises and adds service worker to page
+  window.addEventListener('load', () => {
+    navigator.serviceWorker
+      .register('/service-worker.js')
+      .then(reg => console.log('Service worker registered:', reg.scope))
+      .catch(err => console.error('Registration failed:', err));
+  });
+}