feat(pwa): create native PWA manifest.json

Story 1: Create Native PWA Manifest
- Extract manifest configuration from vite.config.ts to static/manifest.json
- Preserve exact configuration including share_target for Instagram URLs
- Update app.html to reference new manifest.json location
- Validated JSON syntax successfully

All PWA properties maintained:
- Share target: /share route for external app sharing
- Icons: favicon.png in 192x192 and 512x512 sizes
- Display: standalone mode for PWA installation
- Theme colors: white background and theme colors

Refs: docs/plans/MigrateToNativeSvelteKitPWA.md

static/manifest.json

@@ -0,0 +1,31 @@
+{
+  "short_name": "InstaChef",
+  "name": "InstaChef Recipe Saver",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "theme_color": "#ffffff",
+  "background_color": "#ffffff",
+  "icons": [
+    {
+      "src": "/favicon.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/favicon.png",
+      "sizes": "512x512", 
+      "type": "image/png"
+    }
+  ],
+  "share_target": {
+    "action": "/share",
+    "method": "GET",
+    "enctype": "application/x-www-form-urlencoded",
+    "params": {
+      "title": "title",
+      "text": "text",
+      "url": "url"
+    }
+  }
+}