feat: implement browser extension UI with WebAssembly integration

extension/popup/SignMessage.jsx

@@ -0,0 +1,27 @@
+import React, { useState } from 'react';
+
+export default function SignMessage({ onSign, signature, loading }) {
+  const [message, setMessage] = useState('');
+
+  return (
+    <div className="sign-message">
+      <label>Message to sign:</label>
+      <input
+        type="text"
+        placeholder="Enter plaintext message"
+        value={message}
+        onChange={e => setMessage(e.target.value)}
+        style={{width: '100%', marginBottom: 8}}
+      />
+      <button onClick={() => onSign(message)} disabled={!message || loading}>
+        {loading ? 'Signing...' : 'Sign'}
+      </button>
+      {signature && (
+        <div style={{marginTop: '0.5rem'}}>
+          <span>Signature: <code>{signature}</code></span>
+          <button onClick={() => navigator.clipboard.writeText(signature)} style={{marginLeft: 8}}>Copy</button>
+        </div>
+      )}
+    </div>
+  );
+}