fix(character-banner): 배너 등록 다이얼로그에서 언어 기본값을 한국어(ko)로 설정

src/views/Chat/CharacterBanner.vue

@@ -315,7 +315,7 @@ export default {
         imageUrl: '',
         characterId: null,
         bannerId: null,
-        lang: null
+        lang: 'ko'
       },
       languageOptions: [
         { text: '한국어', value: 'ko' },
@@ -412,7 +412,7 @@ export default {
         imageUrl: '',
         characterId: null,
         bannerId: null,
-        lang: null
+        lang: 'ko'
       };
       this.previewImage = null;
       this.searchKeyword = '';
@@ -451,7 +451,7 @@ export default {
         imageUrl: '',
         characterId: null,
         bannerId: null,
-        lang: null
+        lang: 'ko'
       };
       this.previewImage = null;
       this.searchKeyword = '';