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/keybindings.js

@@ -1,5 +1,6 @@
 import { getPref, setPref } from './preferences.js';
 import { getActiveEditor, setOnTabSwitch, setOnModeChange } from './editors.js';
+import { loadScript } from './utils.js';
 
 let currentMode = 'default'; // 'default' | 'vim' | 'emacs'
 let activeAdapter = null;    // vim or emacs adapter instance
@@ -8,19 +9,6 @@ let emacsLoaded = false;
 
 const statusBar = () => document.getElementById('vim-status-bar');
 
-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 ensureVim() {
   if (vimLoaded) return;
   window.monaco = monaco;