feat: add helper utility functions

- Add formatRupiah for currency formatting
- Add formatDate for date formatting
- Add calculateBalance to compute totals
- Add generateId for unique IDs
- Add getCurrentDate for timestamp

utils/helpers.ts

@@ -0,0 +1,43 @@
+import { Transaction, BalanceInfo } from '../types';
+
+export const formatRupiah = (amount: number): string => {
+  return new Intl.NumberFormat('id-ID', {
+    style: 'currency',
+    currency: 'IDR',
+    minimumFractionDigits: 0,
+    maximumFractionDigits: 0,
+  }).format(amount);
+};
+
+export const formatDate = (dateString: string): string => {
+  const date = new Date(dateString);
+  return new Intl.DateTimeFormat('id-ID', {
+    day: 'numeric',
+    month: 'short',
+    year: 'numeric',
+  }).format(date);
+};
+
+export const calculateBalance = (transactions: Transaction[]): BalanceInfo => {
+  const income = transactions
+    .filter((t) => t.type === 'income')
+    .reduce((sum, t) => sum + t.amount, 0);
+  
+  const expense = transactions
+    .filter((t) => t.type === 'expense')
+    .reduce((sum, t) => sum + t.amount, 0);
+  
+  return {
+    total: income - expense,
+    income,
+    expense,
+  };
+};
+
+export const generateId = (): string => {
+  return Date.now().toString(36) + Math.random().toString(36).substr(2);
+};
+
+export const getCurrentDate = (): string => {
+  return new Date().toISOString();
+};