Add browse dashboard, tags, visibility control, export, QR sharing, and embed mode

- Browse dashboard at / with search, framework filter, tag pills, and pagination
- Tags system with autocomplete datalist and per-fiddle tag management
- Listed/unlisted toggle for visibility control (unlisted still accessible via direct URL)
- Export standalone HTML with inlined CSS/JS and framework CDN tags
- QR code modal for sharing fiddle URLs
- Embed mode at /embed/:id for minimal preview-only rendering
- Extract shared loadScript() utility from 4 files into utils.js
- Database schema: listed column, tags and fiddle_tags tables with index

public/js/qr.js

@@ -0,0 +1,42 @@
+import { loadScript } from './utils.js';
+
+let qrLoaded = false;
+
+async function ensureQrLib() {
+  if (qrLoaded) return;
+  await loadScript('https://cdnjs.cloudflare.com/ajax/libs/qrcode-generator/1.4.4/qrcode.min.js');
+  qrLoaded = true;
+}
+
+/**
+ * Show a QR code modal for the given URL.
+ */
+export async function showQrModal(url) {
+  await ensureQrLib();
+
+  const modal = document.getElementById('qr-modal');
+  const canvas = document.getElementById('qr-canvas');
+  const urlDisplay = document.getElementById('qr-url');
+
+  // Generate QR
+  const qr = qrcode(0, 'M');
+  qr.addData(url);
+  qr.make();
+
+  canvas.innerHTML = qr.createSvgTag({ cellSize: 4, margin: 4 });
+  // Make QR SVG white on dark background
+  const svg = canvas.querySelector('svg');
+  if (svg) {
+    svg.style.background = '#fff';
+    svg.style.borderRadius = '4px';
+    svg.style.padding = '8px';
+  }
+
+  urlDisplay.textContent = url;
+  modal.classList.remove('hidden');
+
+  // Close handlers
+  const close = () => modal.classList.add('hidden');
+  document.getElementById('qr-modal-close').onclick = close;
+  modal.onclick = (e) => { if (e.target === modal) close(); };
+}