feat: PWA support with hockey puck icon

Adds full PWA compliance: web app manifest, service worker with
cache-first static / network-first scoreboard strategy, and a
generated hockey puck icon (512, 192, 180, 32px) on the app's
dark navy background. Includes all required meta tags for iOS
standalone mode and a /favicon.ico route.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/static/sw.js

@@ -0,0 +1,49 @@
+const CACHE = 'nhl-scoreboard-v1';
+const PRECACHE = [
+    '/',
+    '/static/styles.css',
+    '/static/script.js',
+    '/static/icon-192x192.png',
+    '/static/icon-512x512.png',
+    '/manifest.json',
+];
+
+self.addEventListener('install', event => {
+    event.waitUntil(caches.open(CACHE).then(c => c.addAll(PRECACHE)));
+    self.skipWaiting();
+});
+
+self.addEventListener('activate', event => {
+    event.waitUntil(
+        caches.keys().then(keys =>
+            Promise.all(keys.filter(k => k !== CACHE).map(k => caches.delete(k)))
+        )
+    );
+    self.clients.claim();
+});
+
+self.addEventListener('fetch', event => {
+    const { pathname } = new URL(event.request.url);
+
+    // Network-first for the live scoreboard API — stale data is useless
+    if (pathname === '/scoreboard') {
+        event.respondWith(
+            fetch(event.request).catch(() => caches.match(event.request))
+        );
+        return;
+    }
+
+    // Cache-first for everything else (static assets, shell)
+    event.respondWith(
+        caches.match(event.request).then(cached => {
+            if (cached) return cached;
+            return fetch(event.request).then(response => {
+                if (response.ok) {
+                    const clone = response.clone();
+                    caches.open(CACHE).then(c => c.put(event.request, clone));
+                }
+                return response;
+            });
+        })
+    );
+});