Add PWA support with service worker, manifest, and icons

sw.js

@@ -0,0 +1,33 @@
+const CACHE_NAME = 'chengyu-v1';
+const ASSETS = [
+  '/',
+  '/index.html',
+  '/style.css',
+  '/app.js',
+  '/data.js',
+  '/manifest.json',
+  '/icons/icon-192.png',
+  '/icons/icon-512.png'
+];
+
+self.addEventListener('install', e => {
+  e.waitUntil(
+    caches.open(CACHE_NAME).then(cache => cache.addAll(ASSETS))
+  );
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', e => {
+  e.waitUntil(
+    caches.keys().then(keys =>
+      Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', e => {
+  e.respondWith(
+    caches.match(e.request).then(cached => cached || fetch(e.request))
+  );
+});