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

public/sw.js

@@ -0,0 +1,16 @@
+/**
+ * Minimal service worker — network pass-through.
+ * Its only job is to make the app installable (browsers require a service
+ * worker to offer "Install app"); no offline caching for now.
+ */
+self.addEventListener("install", (event) => {
+    self.skipWaiting();
+});
+
+self.addEventListener("activate", (event) => {
+    event.waitUntil(clients.claim());
+});
+
+self.addEventListener("fetch", (event) => {
+    event.respondWith(fetch(event.request));
+});