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/manifest.json

@@ -0,0 +1,24 @@
+{
+  "name": "NHL Scoreboard",
+  "short_name": "NHL Scores",
+  "description": "Live NHL game scores ranked by hype",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0f172a",
+  "theme_color": "#0f172a",
+  "orientation": "portrait-primary",
+  "icons": [
+    {
+      "src": "/static/icon-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/static/icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}