feat: add testing infrastructure and improve component feedback

- Added React testing setup with JSDOM
- Added component tests for AddTransactionForm and TransactionTable
- Improved error handling and success messages in components
- Updated test configuration and dependencies
- Added CSS for error and success states

astro.config.mjs

@@ -13,11 +13,8 @@ export default defineConfig({
   }),
   integrations: [react()],
   vite: {
-    resolve: {
-      alias: {
-        'react-dom/server': 'react-dom/server',
-        'react-dom/server.browser': 'react-dom/server',
-      },
+    ssr: {
+      noExternal: ['react-dom/server', 'react-dom/server.browser'],
     },
   },
 });