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/utils.js

@@ -0,0 +1,15 @@
+/**
+ * Load an external script, hiding AMD define to avoid conflicts with Monaco's RequireJS.
+ */
+export function loadScript(src) {
+  return new Promise((resolve, reject) => {
+    if (document.querySelector(`script[src="${src}"]`)) { resolve(); return; }
+    const savedDefine = window.define;
+    window.define = undefined;
+    const s = document.createElement('script');
+    s.src = src;
+    s.onload = () => { window.define = savedDefine; resolve(); };
+    s.onerror = () => { window.define = savedDefine; reject(new Error(`Failed to load ${src}`)); };
+    document.head.appendChild(s);
+  });
+}