Make Crystal an installable web app with an offline shell (#39)

A manifest and icons rendered from icon.svg make the app installable.
The native build emits a service worker that precaches exactly the
built files and passes everything else to the network, so ROMs never
reach its cache. New versions wait for the player's Update, which saves
before reloading. Savegames offers installation, iPhone steps and
persistent storage, and a full disk gets its own save error.

Acceptance on physical iPhone and Android devices remains.

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

playwright.config.ts

@@ -10,9 +10,18 @@ export default defineConfig({
       use: { ...devices["iPhone 13"], defaultBrowserType: "webkit" },
     },
   ],
-  webServer: {
-    command: "npm run dev -- --host 127.0.0.1",
-    url: "http://127.0.0.1:5173",
-    reuseExistingServer: !process.env.CI,
-  },
+  webServer: [
+    {
+      command: "npm run dev -- --host 127.0.0.1",
+      url: "http://127.0.0.1:5173",
+      reuseExistingServer: !process.env.CI,
+    },
+    // The production build, for the service worker and offline checks.
+    {
+      command: "npm run build && npx vite preview --host 127.0.0.1 --port 4173 --strictPort",
+      url: "http://127.0.0.1:4173",
+      reuseExistingServer: !process.env.CI,
+      timeout: 120_000,
+    },
+  ],
 });