Initial commit — AI-powered coding tutor (Professor)

Next.js 16, React 19, Monaco editor, Anthropic SDK, multi-provider AI,
Wandbox Python execution, iframe HTML preview, SQLite auth + session persistence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/editor/HtmlPreview.tsx

@@ -0,0 +1,36 @@
+'use client';
+
+import { useEffect, useState } from 'react';
+
+interface Props {
+  code: string;
+}
+
+export default function HtmlPreview({ code }: Props) {
+  const [previewCode, setPreviewCode] = useState(code);
+
+  // Debounce the preview update to avoid iframe thrashing
+  useEffect(() => {
+    const timer = setTimeout(() => setPreviewCode(code), 500);
+    return () => clearTimeout(timer);
+  }, [code]);
+
+  return (
+    <div className="flex flex-col border-t border-zinc-700">
+      <div className="flex items-center gap-2 border-b border-zinc-700 bg-zinc-900 px-4 py-2">
+        <span className="text-xs font-sans font-medium uppercase tracking-wider text-zinc-400">Preview</span>
+        <div className="ml-auto flex gap-1">
+          <div className="h-2.5 w-2.5 rounded-full bg-red-500/60" />
+          <div className="h-2.5 w-2.5 rounded-full bg-yellow-500/60" />
+          <div className="h-2.5 w-2.5 rounded-full bg-green-500/60" />
+        </div>
+      </div>
+      <iframe
+        srcDoc={previewCode}
+        sandbox="allow-scripts"
+        className="h-48 w-full bg-white"
+        title="HTML Preview"
+      />
+    </div>
+  );
+}