Fix build errors: useCallback syntax and API module check

apps/api/src/index.ts

@@ -78,8 +78,8 @@ app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
   });
 });
 
-// Start server
-if (require.main === module) {
+// Start server (only if running directly, not when imported)
+if (typeof require !== 'undefined' && require.main === module) {
   app.listen(PORT, () => {
     logger.info(`API server started on port ${PORT}`);
   });

apps/web/src/pages/TransactionsPage.tsx

@@ -35,7 +35,7 @@ export default function TransactionsPage() {
 
   const converter = getDefaultConverter();
 
-  const handleSubmit = async (e: React.FormEvent) => {
+  const handleSubmit = useCallback(async (e: React.FormEvent) => {
     e.preventDefault();
     setErrors({});
     setIsProcessing(true);