utilsの整理整頓

1. 関数を機能ごとにファイルに分割

utils/calculate.ts

export const add = (a: number, b: number): number => a + b;
export const subtract = (a: number, b: number): number => a - b;

utils/format.ts

export const formatCurrency = (value: number): string => `$${value.toFixed(2)}`;
export const formatDate = (date: Date): string => date.toISOString().slice(0, 10);

2. index.tsからまとめてエクスポート

utils/index.ts

export * from './calculate';
export * from './format';