Installable as an app (PWA): manifests, offline-only service worker, icons

internal/web/assets/sw.js

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+// The service worker is what lets the site be installed as an app. It caches nothing but the
+// offline page: every request still goes to the network, so nobody sees stale bans or ranks.
+
+const CACHE = "offline-v1";
+const OFFLINE = "/static/offline.html";
+
+self.addEventListener("install", (e) => {
+  e.waitUntil(caches.open(CACHE).then((c) => c.add(OFFLINE)).then(() => self.skipWaiting()));
+});
+
+self.addEventListener("activate", (e) => {
+  e.waitUntil(
+    caches.keys()
+      .then((keys) => Promise.all(keys.filter((k) => k !== CACHE).map((k) => caches.delete(k))))
+      .then(() => self.clients.claim()),
+  );
+});
+
+// Only page loads get the offline page; API calls fail as they always have, and the pages say so.
+self.addEventListener("fetch", (e) => {
+  if (e.request.mode !== "navigate") return;
+  e.respondWith(fetch(e.request).catch(() => caches.match(OFFLINE)));
+});