Add comprehensive i18n localization to vat-return entity

- Convert vat-return constants to i18n-compatible getter functions
- Add 10 translation keys to vatReturn namespace
- Localize 3 vat-return files

Constant conversions:
- getVatReturnReceiptTypeBtnGroup(t)
- getVatReturnTargetTypeBtnGroup(t)
- getVatReturnTaxBtnGroups(t)

Translation keys added:
- vatReturn.issuanceMonth: "발행월" / "Issue Month"
- vatReturn.receiptType: "영수구분" / "Receipt Type"
- vatReturn.issueTarget: "발행대상" / "Issue Target"
- vatReturn.receipt, bill, general
- vatReturn.differenceSettlement, refundSettlement
- vatReturn.taxable, taxExempt
- vatReturn.keyInMerchantError

Localized files:
- filter/list-filter.tsx
- reference-wrap.tsx
- model/contant.ts

All vat-return components now support Korean/English language switching.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/locales/ko.json

@@ -852,9 +852,17 @@
         "issueInfo": "발행 정보",
         "issueTargetDate": "발행대상일자",
         "issueDate": "발행일자",
+        "issuanceMonth": "발행월",
         "subject": "적요",
         "issueTarget": "발행대상",
         "receiptType": "영수구분",
+        "receipt": "영수",
+        "bill": "청구",
+        "general": "일반",
+        "differenceSettlement": "차액정산",
+        "refundSettlement": "환급정산",
+        "taxable": "과세",
+        "taxExempt": "면세",
         "supplierInfo": "공급자 정보",
         "receiverInfo": "공급받는 자 정보",
         "taxExemptionType": "거래 과세/면세 구분",
@@ -868,6 +876,7 @@
         "applicationDate": "신청일",
         "emailAddress": "메일주소",
         "referenceDataNote": "참고용 자료이므로 반드시 내부 회계자료 등과\n비교 후 사용해 주세요.",
-        "result": "결과"
+        "result": "결과",
+        "keyInMerchantError": "[9997] 키인가맹점이 아닙니다.<br/>NICEPAY로 문의하세요."
     }
 }