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/public/icon.svg

@@ -0,0 +1,46 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" fill="none">
+  <!-- Background circle with emerald gradient -->
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#10b981;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#059669;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  
+  <!-- Background -->
+  <circle cx="256" cy="256" r="256" fill="url(#grad)"/>
+  
+  <!-- Pantry shelves icon (simplified cabinet with items) -->
+  <g transform="translate(106, 96)">
+    <!-- Cabinet outline -->
+    <rect x="0" y="0" width="300" height="320" rx="12" fill="none" stroke="white" stroke-width="8"/>
+    
+    <!-- Top shelf -->
+    <line x1="0" y1="80" x2="300" y2="80" stroke="white" stroke-width="6"/>
+    
+    <!-- Middle shelf -->
+    <line x1="0" y1="160" x2="300" y2="160" stroke="white" stroke-width="6"/>
+    
+    <!-- Bottom shelf -->
+    <line x1="0" y1="240" x2="300" y2="240" stroke="white" stroke-width="6"/>
+    
+    <!-- Top shelf items - jars -->
+    <circle cx="60" cy="40" r="25" fill="white" opacity="0.9"/>
+    <circle cx="150" cy="40" r="25" fill="white" opacity="0.9"/>
+    <circle cx="240" cy="40" r="25" fill="white" opacity="0.9"/>
+    
+    <!-- Middle shelf items - boxes -->
+    <rect x="35" y="105" width="50" height="40" rx="4" fill="white" opacity="0.9"/>
+    <rect x="125" y="105" width="50" height="40" rx="4" fill="white" opacity="0.9"/>
+    <rect x="215" y="105" width="50" height="40" rx="4" fill="white" opacity="0.9"/>
+    
+    <!-- Bottom shelf items - cans -->
+    <rect x="35" y="185" width="50" height="45" rx="6" fill="white" opacity="0.9"/>
+    <rect x="125" y="185" width="50" height="45" rx="6" fill="white" opacity="0.9"/>
+    <rect x="215" y="185" width="50" height="45" rx="6" fill="white" opacity="0.9"/>
+    
+    <!-- Very bottom items - larger containers -->
+    <rect x="45" y="260" width="70" height="50" rx="6" fill="white" opacity="0.9"/>
+    <rect x="185" y="260" width="70" height="50" rx="6" fill="white" opacity="0.9"/>
+  </g>
+</svg>