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

src/styles/global.css

@@ -213,6 +213,58 @@ tbody tr:hover {
   border-color: #f5c6cb;
 }
 
+.error-message {
+  color: #dc3545;
+  background-color: #f8d7da;
+  border: 1px solid #f5c6cb;
+  border-radius: 4px;
+  padding: 8px 12px;
+  margin-bottom: 15px;
+  font-size: 0.9em;
+}
+
+.success-message {
+  color: #198754;
+  background-color: #d1e7dd;
+  border: 1px solid #badbcc;
+  border-radius: 4px;
+  padding: 8px 12px;
+  margin-bottom: 15px;
+  font-size: 0.9em;
+}
+
+.button-group {
+  display: flex;
+  gap: 10px;
+  margin-top: 15px;
+}
+
+.cancel-btn {
+  background-color: #6c757d;
+  color: white;
+  padding: 8px 15px;
+  border: none;
+  border-radius: 3px;
+  cursor: pointer;
+}
+
+.cancel-btn:hover {
+  background-color: #5a6268;
+}
+
+.cancel-btn:disabled {
+  background-color: #6c757d;
+  opacity: 0.65;
+  cursor: not-allowed;
+}
+
+.help-text {
+  display: block;
+  margin-top: 4px;
+  color: #6c757d;
+  font-size: 0.85em;
+}
+
 /* Basic Responsive */
 @media (max-width: 768px) {
   .dashboard-layout {