add offline support

public/offline-worker.js

@@ -0,0 +1,38 @@
+'use strict';
+// Serve app files and individually selected route snapshots independently (v2).
+importScripts('/js/offline-storage.js');
+
+self.addEventListener('install', event => event.waitUntil(self.skipWaiting()));
+self.addEventListener('activate', event => event.waitUntil(self.clients.claim()));
+
+/** Audio seeks need partial responses even when the entire file is stored locally. */
+async function rangedResponse(response, range) {
+  const bytes = await response.arrayBuffer();
+  const match = /^bytes=(\d*)-(\d*)$/.exec(range);
+  if (!match || (!match[1] && !match[2])) return new Response(null, { status: 416, headers: { 'Content-Range': `bytes */${bytes.byteLength}` } });
+  const start = match[1] ? Number(match[1]) : Math.max(0, bytes.byteLength - Number(match[2]));
+  const end = match[1] ? (match[2] ? Math.min(Number(match[2]), bytes.byteLength - 1) : bytes.byteLength - 1) : bytes.byteLength - 1;
+  if (start > end || start >= bytes.byteLength) return new Response(null, { status: 416, headers: { 'Content-Range': `bytes */${bytes.byteLength}` } });
+  const headers = new Headers(response.headers);
+  headers.set('Content-Range', `bytes ${start}-${end}/${bytes.byteLength}`);
+  headers.set('Content-Length', String(end - start + 1));
+  headers.set('Accept-Ranges', 'bytes');
+  return new Response(bytes.slice(start, end + 1), { status: 206, headers });
+}
+
+self.addEventListener('fetch', event => {
+  const request = event.request;
+  const url = new URL(request.url);
+  if (request.method !== 'GET' || url.origin !== self.location.origin
+    || request.headers.has('X-Wegwichtel-Offline-Download')
+    || url.pathname === '/api/offline-manifest') return;
+  event.respondWith((async () => {
+    // A complete snapshot is cache-first, avoiding any wait for a failing mobile network.
+    const cached = await self.Wegwichtel.OfflineStore.match(request.url).catch(() => null);
+    if (cached) {
+      const range = request.headers.get('Range');
+      return range ? rangedResponse(cached, range) : cached;
+    }
+    return fetch(request);
+  })());
+});