Initial commit: code playground with multi-framework support

Express + SQLite backend with Monaco editor frontend.
Supports HTML/CSS/JS, TypeScript, React (JSX/TSX), Vue SFC,
and Svelte with live preview, console output, save/fork/share.

Includes CSS preprocessors (SCSS, Less), framework-specific
compilation (Babel, TypeScript, Svelte compiler), and
CDN-loaded runtime libraries for preview rendering.

public/js/preprocessors.js

@@ -0,0 +1,54 @@
+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');
+  sassLoaded = true;
+}
+
+async function ensureLess() {
+  if (lessLoaded) return;
+  window.less = { env: 'production' };
+  await loadScript('https://cdnjs.cloudflare.com/ajax/libs/less.js/4.2.0/less.min.js');
+  lessLoaded = true;
+}
+
+export async function compileCss(code, type) {
+  if (type === 'css') return code;
+
+  if (type === 'scss') {
+    await ensureSass();
+    return new Promise((resolve, reject) => {
+      Sass.compile(code, (result) => {
+        if (result.status === 0) resolve(result.text);
+        else reject(new Error(result.formatted || result.message));
+      });
+    });
+  }
+
+  if (type === 'less') {
+    await ensureLess();
+    try {
+      const result = await less.render(code);
+      return result.css;
+    } catch (e) {
+      throw new Error(e.message);
+    }
+  }
+
+  return code;
+}