Add service worker

public/sw.js

@@ -0,0 +1,49 @@
+// Service worker: precache the app shell and engine so the app opens offline.
+// Same-origin only; model + voice files are cached by phonon-tts itself.
+const VERSION = '__VERSION__';
+const PRECACHE = __PRECACHE__;
+const CACHE = 'reader-shell-' + VERSION;
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(CACHE).then((c) => c.addAll(PRECACHE)).then(() => self.skipWaiting())
+  );
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys()
+      .then((keys) => Promise.all(
+        keys.filter((k) => k.startsWith('reader-shell-') && k !== CACHE).map((k) => caches.delete(k))
+      ))
+      .then(() => self.clients.claim())
+  );
+});
+
+self.addEventListener('fetch', (event) => {
+  const req = event.request;
+  if (req.method !== 'GET') return;
+  const url = new URL(req.url);
+  if (url.origin !== self.location.origin) return;
+
+  // Page navigations (including share-target URLs with ?text=...): serve the
+  // cached shell, so it works offline; the query string is read client-side.
+  if (req.mode === 'navigate') {
+    event.respondWith(
+      caches.open(CACHE).then(async (c) =>
+        (await c.match('./index.html')) || (await c.match('./')) || fetch(req)
+      )
+    );
+    return;
+  }
+
+  event.respondWith(
+    caches.open(CACHE).then(async (c) => {
+      const hit = await c.match(req, { ignoreSearch: true });
+      if (hit) return hit;
+      const res = await fetch(req);
+      if (res.ok) c.put(req, res.clone());
+      return res;
+    })
+  );
+});