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/scripts/generate-icons.js

@@ -0,0 +1,70 @@
+#!/usr/bin/env node
+import { readFile, writeFile } from 'fs/promises';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+import sharp from 'sharp';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const publicDir = join(__dirname, '..', 'public');
+const svgPath = join(publicDir, 'icon.svg');
+
+const sizes = [
+  { size: 192, name: 'icon-192x192.png', maskable: false },
+  { size: 512, name: 'icon-512x512.png', maskable: false },
+  { size: 192, name: 'icon-192x192-maskable.png', maskable: true },
+  { size: 512, name: 'icon-512x512-maskable.png', maskable: true },
+];
+
+async function generateIcons() {
+  console.log('Reading SVG icon...');
+  const svgBuffer = await readFile(svgPath);
+  
+  for (const { size, name, maskable } of sizes) {
+    console.log(`Generating ${name}...`);
+    
+    let buffer;
+    if (maskable) {
+      // Maskable icons need safe zone padding (80% of icon in center)
+      // Create a transparent canvas with padding
+      const paddedSize = size;
+      const iconSize = Math.floor(size * 0.8);
+      const offset = Math.floor((paddedSize - iconSize) / 2);
+      
+      // Resize SVG to icon size
+      const iconBuffer = await sharp(svgBuffer)
+        .resize(iconSize, iconSize)
+        .png()
+        .toBuffer();
+      
+      // Create transparent background and composite
+      buffer = await sharp({
+        create: {
+          width: paddedSize,
+          height: paddedSize,
+          channels: 4,
+          background: { r: 0, g: 0, b: 0, alpha: 0 }
+        }
+      })
+        .composite([{
+          input: iconBuffer,
+          top: offset,
+          left: offset
+        }])
+        .png()
+        .toBuffer();
+    } else {
+      // Regular icon - full size
+      buffer = await sharp(svgBuffer)
+        .resize(size, size)
+        .png()
+        .toBuffer();
+    }
+    
+    await writeFile(join(publicDir, name), buffer);
+    console.log(`✓ ${name}`);
+  }
+  
+  console.log('\n✅ All icons generated successfully!');
+}
+
+generateIcons().catch(console.error);