sw.js

const CACHE_NAME = 'stack-sim-v12';
const ASSETS = [
  '/',
  '/index.html',
  '/css/style.css',
  '/js/script.js',
  '/icons/favicon-16x16.png',
  '/icons/favicon-32x32.png',
  '/icons/android-chrome-192x192.png',
  '/icons/android-chrome-512x512.png',
  '/icons/apple-touch-icon.png',
  '/manifest.json'
];

self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open(CACHE_NAME).then(cache => cache.addAll(ASSETS))
  );
  self.skipWaiting();
});

self.addEventListener('activate', (event) => {
  event.waitUntil(
    caches.keys().then(keys => Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k))))
  );
  self.clients.claim();
});

self.addEventListener('fetch', (event) => {
  const req = event.request;
  const url = new URL(req.url);

  if (req.mode === 'navigate' || (req.headers.get('accept') || '').includes('text/html')) {
    event.respondWith(
      fetch(req).then(resp => {
        const copy = resp.clone();
        caches.open(CACHE_NAME).then(cache => cache.put(req, copy));
        return resp;
      }).catch(() => caches.match(req).then(m => m || caches.match('/index.html')))
    );
    return;
  }

  if (url.pathname.startsWith('/css/') || url.pathname.startsWith('/js/') || url.pathname.startsWith('/icons/')) {
    event.respondWith(
      caches.match(req).then(cached => cached || fetch(req).then(resp => {
        const copy = resp.clone();
        caches.open(CACHE_NAME).then(cache => cache.put(req, copy));
        return resp;
      }))
    );
    return;
  }
});