Offline shell (precache, offline notice) and accessibility layer (skip link, button names, sheet focus and Escape)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/sw.js

@@ -1,11 +1,22 @@
 // App-shell cache only; Seafile/API traffic always goes to the network.
-const CACHE = 'hub-v6';
+const CACHE = 'hub-v7';
 const SHELL = ['/icon.svg', '/manifest.webmanifest', '/icons/icon-192.png', '/icons/apple-touch-icon.png'];
 
-self.addEventListener('install', e => {
-  e.waitUntil(caches.open(CACHE).then(c => c.addAll(SHELL)));
-  self.skipWaiting();
-});
+// Install: keep the icons, and (when signed in) the page itself with the scripts and styles it names, so the app opens offline.
+async function precache() {
+  const c = await caches.open(CACHE);
+  await c.addAll(SHELL);
+  try {
+    const r = await fetch('/', { credentials: 'same-origin', redirect: 'manual' });
+    if (r.ok) {
+      const html = await r.clone().text();
+      await c.put('/', r);
+      const urls = [...html.matchAll(/(?:src|href)="(\/[^"]+\.(?:js|css)(?:\?[^"]*)?)"/g)].map(m => m[1]);
+      await Promise.all(urls.map(u => c.add(u).catch(() => {})));
+    }
+  } catch { /* signed out or offline: it is cached the first time it is used */ }
+}
+self.addEventListener('install', e => { e.waitUntil(precache()); self.skipWaiting(); });
 self.addEventListener('activate', e => {
   e.waitUntil(caches.keys().then(ks => Promise.all(ks.filter(k => k !== CACHE).map(k => caches.delete(k)))));
   self.clients.claim();