feat: generate PWA icons and assets (#33)

- Create icon.svg with pantry shelves design
- Generate icon-192x192.png and icon-512x512.png
- Generate maskable variants for better Android support
- Create favicon.ico and apple-touch-icon.png
- Generate placeholder screenshots (mobile + desktop)
- Add icon generation scripts using sharp
- Add npm script for easy regeneration

Icon design features:
- Emerald gradient background (#10b981)
- Pantry shelves with jars, boxes, and cans
- Clean, recognizable silhouette
- Works at all sizes

Closes #33

app/package.json

@@ -7,7 +7,8 @@
     "dev": "nuxt dev",
     "generate": "nuxt generate",
     "preview": "nuxt preview",
-    "postinstall": "nuxt prepare"
+    "postinstall": "nuxt prepare",
+    "generate:icons": "node scripts/generate-icons.js && node scripts/generate-screenshots.js"
   },
   "dependencies": {
     "@nuxt/fonts": "^0.13.0",
@@ -20,6 +21,7 @@
   },
   "devDependencies": {
     "@nuxtjs/tailwindcss": "^6.14.0",
-    "@vite-pwa/nuxt": "^1.1.1"
+    "@vite-pwa/nuxt": "^1.1.1",
+    "sharp": "^0.34.5"
   }
 }