Fix NaN percentage display

Ensure percentage displays 0 instead of NaN when budget target is zero.

src/utils/formatters.ts

@@ -8,5 +8,7 @@ export const formatCurrency = (amount: number): string => {
 };
 
 export const calculatePercentage = (spent: number, target: number): number => {
+  // 타겟이 0이면 0%를 반환하도록 수정
+  if (target === 0 || isNaN(target)) return 0;
   return Math.min(Math.round(spent / target * 100), 100);
 };