feat: add form validation

- Add Alert for validation errors
- Check amount and description
- Validate positive number

components/TransactionForm.tsx

@@ -6,6 +6,7 @@ import {
   TouchableOpacity,
   StyleSheet,
   ScrollView,
+  Alert,
 } from 'react-native';
 import { COLORS, FONTS } from '../constants/theme';
 import { Transaction, CATEGORIES } from '../types';
@@ -22,11 +23,20 @@ export const TransactionForm: React.FC<TransactionFormProps> = ({ onAdd }) => {
   const [category, setCategory] = useState('lainnya');
 
   const handleSubmit = () => {
-    if (!amount || !description) return;
+    if (!amount || !description) {
+      Alert.alert('Error', 'Mohon isi jumlah dan deskripsi');
+      return;
+    }
+    
+    const numAmount = parseInt(amount, 10);
+    if (isNaN(numAmount) || numAmount <= 0) {
+      Alert.alert('Error', 'Jumlah harus angka positif');
+      return;
+    }
     
     const transaction: Transaction = {
       id: generateId(),
-      amount: parseInt(amount, 10),
+      amount: numAmount,
       description,
       type,
       category,