Installable as an app (PWA): manifest, pixel-art icons, offline service worker

- manifest.webmanifest (full screen, any orientation), icons 192/512/maskable/apple-touch
- sw.js: network first, cache as offline fallback, the savegame API is never cached
- registered only over HTTPS or on localhost; nginx serves the manifest type
- README: installing, HTTPS requirement

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

www/sw.js

@@ -0,0 +1,45 @@
+// DungeonCraft service worker: makes the game installable and playable offline.
+// Network first (updates arrive at once), the cache is the fallback when offline.
+// The savegame service (/api/) is never cached.
+const CACHE = "dungeoncraft-v1";
+const CORE = [
+  "./",
+  "index.html",
+  "mq_js_bundle.js",
+  "dc_plugin.js",
+  "dungeoncraft.wasm",
+  "manifest.webmanifest",
+  "icons/icon-192.png",
+  "icons/icon-512.png",
+];
+
+self.addEventListener("install", (event) => {
+  event.waitUntil(caches.open(CACHE).then((c) => c.addAll(CORE)).then(() => self.skipWaiting()));
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches.keys()
+      .then((keys) => Promise.all(keys.filter((k) => k !== CACHE).map((k) => caches.delete(k))))
+      .then(() => self.clients.claim())
+  );
+});
+
+self.addEventListener("fetch", (event) => {
+  const req = event.request;
+  const url = new URL(req.url);
+  if (req.method !== "GET" || url.origin !== self.location.origin || url.pathname.includes("/api/")) {
+    return;
+  }
+  event.respondWith(
+    fetch(req)
+      .then((res) => {
+        if (res.ok) {
+          const copy = res.clone();
+          caches.open(CACHE).then((c) => c.put(req, copy));
+        }
+        return res;
+      })
+      .catch(() => caches.match(req).then((hit) => hit || caches.match("index.html")))
+  );
+});