fix: icônes PNG + fetch handler pour déclencher le prompt PWA

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/scripts/gen-icons.js

@@ -0,0 +1,17 @@
+// Génère icon-192.png et icon-512.png depuis icon.svg
+import sharp from 'sharp';
+import { readFileSync } from 'fs';
+import { fileURLToPath } from 'url';
+import { dirname, resolve } from 'path';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const svgPath   = resolve(__dirname, '../public/icon.svg');
+const svg       = readFileSync(svgPath);
+
+for (const size of [192, 512]) {
+  await sharp(svg)
+    .resize(size, size)
+    .png()
+    .toFile(resolve(__dirname, `../public/icon-${size}.png`));
+  console.log(`icon-${size}.png generated`);
+}