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>

static/icon.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" rx="96" fill="#2b6a3d"/>
+  <g fill="none" stroke="#ffffff" stroke-width="16" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M128 192 L256 320 L384 192" />
+    <circle cx="256" cy="256" r="128" />
+  </g>
+  <text x="256" y="460" font-family="system-ui,sans-serif" font-size="80" font-weight="700" fill="#ffffff" text-anchor="middle">Koch</text>
+</svg>

static/manifest.webmanifest

@@ -0,0 +1,19 @@
+{
+  "name": "Kochwas",
+  "short_name": "Kochwas",
+  "description": "Persönliches Rezeptbuch — lokal, einheitlich, küchentauglich",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#f8faf8",
+  "theme_color": "#2b6a3d",
+  "lang": "de",
+  "orientation": "portrait",
+  "icons": [
+    {
+      "src": "/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    }
+  ]
+}