fix: TagManagePage service_name null 인덱싱 타입 에러 수정 (#45)

- service_name이 null일 때 Record 인덱스로 사용 불가 (TS2538)
- null 폴백 처리: item.service_name ?? "미지정"

Closes #45

react/src/features/tag/pages/TagManagePage.tsx

@@ -78,7 +78,8 @@ export default function TagManagePage() {
       const counts: Record<string, number> = {};
       let total = 0;
       for (const item of items) {
-        counts[item.service_name] = (counts[item.service_name] ?? 0) + 1;
+        const key = item.service_name ?? "미지정";
+        counts[key] = (counts[key] ?? 0) + 1;
         total++;
       }
       counts["전체"] = total;