Make the app installable (PWA metadata)

- manifest.webmanifest with name, standalone display, theme color
- PNG icons 192/512 + maskable variant + apple-touch-icon (from the
  Poké Ball favicon)
- minimal pass-through service worker (installability requirement)
- apple/mobile web app meta tags

src/layouts/BaseLayout.astro

@@ -26,6 +26,16 @@ if (themePreference === "Dark") {
         <meta charset="utf-8" />
         <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
         <link rel="icon" href="/favicon.ico" />
+        <link rel="manifest" href="/manifest.webmanifest" />
+        <meta name="theme-color" content="#ee1515" />
+        <link rel="apple-touch-icon" href="/apple-touch-icon.png" />
+        <meta name="apple-mobile-web-app-capable" content="yes" />
+        <meta
+            name="apple-mobile-web-app-status-bar-style"
+            content="black-translucent"
+        />
+        <meta name="apple-mobile-web-app-title" content="PokeShare" />
+        <meta name="mobile-web-app-capable" content="yes" />
         <meta name="viewport" content="width=device-width" />
         <meta name="generator" content={Astro.generator} />
         <title>PokeShare</title>
@@ -59,5 +69,11 @@ if (themePreference === "Dark") {
             import "basecoat-css/tabs";
             import "basecoat-css/dropdown-menu";
         </script>
+        <script is:inline>
+            // Register the service worker so the browser offers "Install app"
+            if ("serviceWorker" in navigator) {
+                navigator.serviceWorker.register("/sw.js");
+            }
+        </script>
     </body>
 </html>