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/manifest.webmanifest

@@ -0,0 +1,17 @@
+{
+  "name": "DungeonCraft",
+  "short_name": "DungeonCraft",
+  "description": "Dig. Build. Defend. Conquer. A dungeon building and defence game.",
+  "start_url": "./",
+  "scope": "./",
+  "display": "fullscreen",
+  "display_override": ["fullscreen", "standalone"],
+  "orientation": "any",
+  "background_color": "#080608",
+  "theme_color": "#080608",
+  "icons": [
+    { "src": "icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "icons/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "icons/maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}