fix: replace Buffer.from().toString('base64') with btoa helper — Buffer not available in Expo RN

mobile/app/(tabs)/import.tsx

@@ -228,10 +228,21 @@ async function fetchWheelBase64(instanceUrl: string): Promise<string> {
   const resp = await fetch(wheelUrl);
   if (!resp.ok) throw new Error(`Could not download Bincio engine (${resp.status}). Is the instance running?`);
   const buf = await resp.arrayBuffer();
-  _cachedWheelBase64 = Buffer.from(buf).toString('base64');
+  _cachedWheelBase64 = arrayBufferToBase64(buf);
   return _cachedWheelBase64;
 }
 
+function arrayBufferToBase64(buf: ArrayBuffer): string {
+  const bytes = new Uint8Array(buf);
+  let binary = '';
+  // Process in chunks to avoid spread-operator stack overflow on large arrays.
+  const CHUNK = 8192;
+  for (let i = 0; i < bytes.length; i += CHUNK) {
+    binary += String.fromCharCode(...(bytes.subarray(i, i + CHUNK) as unknown as number[]));
+  }
+  return btoa(binary);
+}
+
 // ── Styles ───────────────────────────────────────────────────────────────────
 
 const styles = StyleSheet.create({