feat: add proper favicon, apple-touch-icon, webmanifest and PWA icons

Generate PNGs from the trophy SVG (dark #040d08 background, centred):
- favicon.svg       — primary, all modern browsers
- favicon-32x32.png — 32×32 fallback for older browsers
- apple-touch-icon.png — 180×180 for iOS home screen
- icon-192x192.png / icon-512x512.png — webmanifest / PWA

app/manifest.ts provides /manifest.webmanifest via Next.js file convention.
layout.tsx metadata wires up all icon sizes via the icons API.

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

app/layout.tsx

@@ -11,7 +11,13 @@ export const metadata: Metadata = {
   title: { default: 'World Cup', template: '%s · World Cup' },
   description: 'Comprehensive World Cup statistics from 1930 to 2026',
   icons: {
-    icon: '/favicon.svg',
+    icon: [
+      { url: '/favicon.svg', type: 'image/svg+xml' },
+      { url: '/favicon-32x32.png', sizes: '32x32', type: 'image/png' },
+    ],
+    apple: [
+      { url: '/apple-touch-icon.png', sizes: '180x180', type: 'image/png' },
+    ],
   },
 }