Fix Vercel CI: serve static redirect page instead of Python function

Vercel auto-detected app.py and failed trying to run it as a serverless
function. Added vercel.json pointing to a static public/ directory, and
a minimal index.html that directs visitors to Hugging Face Spaces where
the app actually runs.

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

public/index.html

@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <title>Lean 4 Proof Assistant</title>
+  <style>
+    body { font-family: sans-serif; display: flex; align-items: center; justify-content: center; height: 100vh; margin: 0; background: #f5f5f5; }
+    .card { background: white; padding: 2rem 3rem; border-radius: 12px; box-shadow: 0 2px 12px rgba(0,0,0,0.1); text-align: center; }
+    a { color: #f97316; font-weight: 600; text-decoration: none; }
+    a:hover { text-decoration: underline; }
+  </style>
+</head>
+<body>
+  <div class="card">
+    <h1>Lean 4 Proof Assistant</h1>
+    <p>This app runs on <strong>Hugging Face Spaces</strong>.</p>
+    <p><a href="https://huggingface.co/spaces">Open the app on Hugging Face Spaces &rarr;</a></p>
+  </div>
+</body>
+</html>

vercel.json

@@ -0,0 +1,5 @@
+{
+  "framework": null,
+  "buildCommand": null,
+  "outputDirectory": "public"
+}