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/en.json

@@ -848,9 +848,17 @@
         "issueInfo": "Issue Information",
         "issueTargetDate": "Issue Target Date",
         "issueDate": "Issue Date",
+        "issuanceMonth": "Issue Month",
         "subject": "Subject",
         "issueTarget": "Issue Target",
         "receiptType": "Receipt Type",
+        "receipt": "Receipt",
+        "bill": "Bill",
+        "general": "General",
+        "differenceSettlement": "Difference Settlement",
+        "refundSettlement": "Refund Settlement",
+        "taxable": "Taxable",
+        "taxExempt": "Tax Exempt",
         "supplierInfo": "Supplier Information",
         "receiverInfo": "Receiver Information",
         "taxExemptionType": "Tax/Exemption Type",
@@ -864,6 +872,7 @@
         "applicationDate": "Application Date",
         "emailAddress": "Email Address",
         "referenceDataNote": "Please compare with internal accounting data\nbefore using this reference material.",
-        "result": "Result"
+        "result": "Result",
+        "keyInMerchantError": "[9997] Not a Key-In merchant.<br/>Please contact NICEPAY."
     }
 }