Add payment method selection

Adds a payment method selection (Credit Card, Cash) to the expense form and includes a line separator. Also requests to add a graph showing the proportion of credit card and cash usage in expense analytics, but this part is not implemented in this commit.

src/hooks/transactions/storageUtils.ts

@@ -19,11 +19,31 @@ export const loadTransactionsFromStorage = (): Transaction[] => {
           if (transaction.type === 'expense') {
             // 기존 카테고리명 변환
             if (transaction.category === '식비') {
-              return { ...transaction, category: '음식' };
+              return { 
+                ...transaction, 
+                category: '음식',
+                paymentMethod: transaction.paymentMethod || '신용카드' // 기존 데이터에 없으면 기본값 추가
+              };
             } else if (transaction.category === '생활비') {
-              return { ...transaction, category: '쇼핑' };
+              return { 
+                ...transaction, 
+                category: '쇼핑',
+                paymentMethod: transaction.paymentMethod || '신용카드' // 기존 데이터에 없으면 기본값 추가
+              };
             } else if (!EXPENSE_CATEGORIES.includes(transaction.category)) {
-              return { ...transaction, category: '쇼핑' }; // 지원되지 않는 카테고리는 '쇼핑'으로
+              return { 
+                ...transaction, 
+                category: '쇼핑',
+                paymentMethod: transaction.paymentMethod || '신용카드' // 기존 데이터에 없으면 기본값 추가
+              }; // 지원되지 않는 카테고리는 '쇼핑'으로
+            }
+            
+            // 기존 데이터에 paymentMethod가 없으면 기본값 추가
+            if (!transaction.paymentMethod) {
+              return {
+                ...transaction,
+                paymentMethod: '신용카드'
+              };
             }
           }
           return transaction;