feat(pwa): add web manifest, SVG icon, and offline service worker

Service worker caches app shell + images for offline recipe access in the kitchen.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app.html

@@ -4,6 +4,12 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
     <meta name="theme-color" content="#2b6a3d" />
+    <link rel="manifest" href="/manifest.webmanifest" />
+    <link rel="icon" type="image/svg+xml" href="/icon.svg" />
+    <link rel="apple-touch-icon" href="/icon.svg" />
+    <meta name="apple-mobile-web-app-capable" content="yes" />
+    <meta name="apple-mobile-web-app-status-bar-style" content="default" />
+    <meta name="apple-mobile-web-app-title" content="Kochwas" />
     <title>Kochwas</title>
     %sveltekit.head%
   </head>