Add PWA support for iOS home screen install

- Web app manifest (standalone display, NOMAD branding, icons)
- Service worker with Workbox caching (map tiles, API, uploads, CDN)
- SVG app icon + PNG generation script via sharp
- Apple meta tags (web-app-capable, black-translucent status bar)
- Dynamic theme-color for dark/light mode
- Safe area insets for notch devices

client/scripts/generate-icons.mjs

@@ -0,0 +1,29 @@
+/**
+ * Generates PNG icons for PWA from the master SVG icon.
+ * Run: node scripts/generate-icons.mjs
+ * Called automatically via the "prebuild" npm script.
+ */
+import { readFileSync } from 'fs';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+import sharp from 'sharp';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const iconsDir = join(__dirname, '..', 'public', 'icons');
+const svgBuffer = readFileSync(join(iconsDir, 'icon.svg'));
+
+const sizes = [
+  { name: 'apple-touch-icon-180x180.png', size: 180 },
+  { name: 'icon-192x192.png', size: 192 },
+  { name: 'icon-512x512.png', size: 512 },
+];
+
+for (const { name, size } of sizes) {
+  await sharp(svgBuffer, { density: 300 })
+    .resize(size, size)
+    .png({ compressionLevel: 9 })
+    .toFile(join(iconsDir, name));
+  console.log(`  \u2713 ${name} (${size}x${size})`);
+}
+
+console.log('PWA icons generated.');