Add i18n localization to calendar-month filter component

- Add translation keys to filter namespace:
  - filter.periods.currentMonth: Current month period option
  - filter.periods.2months: 2 months period option
  - filter.selectDate: Date selection placeholder and alt text

- Localize shared/ui/filter/calendar-month.tsx:
  - Replace hardcoded "조회기간" with filter.period
  - Replace hardcoded "당월" with filter.periods.currentMonth
  - Replace hardcoded "2개월" with filter.periods.2months
  - Replace hardcoded "3개월" with filter.periods.3months
  - Replace hardcoded "직접입력" with filter.periods.custom
  - Replace hardcoded "날짜 선택" with filter.selectDate

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

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

src/locales/ko.json

@@ -91,11 +91,14 @@
           "issueNumber": "발행번호"
         },
         "periods": {
+          "currentMonth": "당월",
           "1month": "1개월",
+          "2months": "2개월",
           "3months": "3개월",
           "6months": "6개월",
           "custom": "직접입력"
         },
+        "selectDate": "날짜 선택",
         "transactionType": "거래구분",
         "transactionTypes": {
           "all": "전체",