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/index.html

@@ -3,7 +3,14 @@
 <head>
   <meta charset="utf-8">
   <title>DungeonCraft</title>
-  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="theme-color" content="#080608">
+  <meta name="mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black">
+  <link rel="manifest" href="manifest.webmanifest">
+  <link rel="icon" type="image/png" href="icons/icon-192.png">
+  <link rel="apple-touch-icon" href="icons/apple-touch-icon.png">
   <style>
     html, body { margin: 0; padding: 0; width: 100%; height: 100%; overflow: hidden; background: #080608; }
     canvas { display: block; width: 100%; height: 100%; outline: none; image-rendering: pixelated; }
@@ -17,6 +24,10 @@
   <script>
     document.getElementById("glcanvas").focus();
     load("dungeoncraft.wasm");
+    // installable app (PWA): only over HTTPS or on localhost
+    if ("serviceWorker" in navigator && (location.protocol === "https:" || location.hostname === "localhost")) {
+      navigator.serviceWorker.register("sw.js").catch(function () {});
+    }
   </script>
 </body>
 </html>