Refactor: Split Index page

Refactor Index.tsx into smaller, more manageable components.

src/utils/formatters.ts

@@ -0,0 +1,12 @@
+
+export const formatCurrency = (amount: number): string => {
+  return new Intl.NumberFormat('ko-KR', {
+    style: 'currency',
+    currency: 'KRW',
+    maximumFractionDigits: 0
+  }).format(amount);
+};
+
+export const calculatePercentage = (spent: number, target: number): number => {
+  return Math.min(Math.round(spent / target * 100), 100);
+};