Fix: Resolve TypeScript errors

The type `Transaction` was missing the `localTimestamp` property, causing TypeScript errors in `downloadTransaction.ts` and `uploadTransaction.ts`. This commit adds the `localTimestamp` property to the `Transaction` interface to resolve these errors.

src/contexts/budget/types.ts

@@ -40,5 +40,7 @@ export interface Transaction {
   date: string;
   category: string;
   type: 'income' | 'expense';
-  notes?: string; // notes 필드를 옵셔널로 추가
+  notes?: string;
+  localTimestamp?: string; // 로컬 수정 타임스탬프 추가
+  serverTimestamp?: string; // 서버 타임스탬프 추가
 }