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/preprocessors.js

@@ -1,19 +1,8 @@
+import { loadScript } from './utils.js';
+
 let sassLoaded = false;
 let lessLoaded = false;
 
-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);
-  });
-}
-
 async function ensureSass() {
   if (sassLoaded) return;
   await loadScript('https://cdnjs.cloudflare.com/ajax/libs/sass.js/0.11.1/sass.sync.min.js');