feat(l10n): 캐릭터 생성 화면 하드코딩 텍스트 l10n 적용

- newCharacterTitle, soldButton 키 추가 (app_en.arb, app_ko.arb)
- new_character_screen.dart 하드코딩 텍스트를 L10n 함수로 변경
- 관련 테스트 업데이트 (widget_test.dart, new_character_screen_test.dart)

변경 내역:
- "Progress Quest - New Character" → L10n.newCharacterTitle
- "Sold!" → L10n.soldButton

lib/src/features/new_character/new_character_screen.dart

@@ -200,7 +200,7 @@ class _NewCharacterScreenState extends State<NewCharacterScreen> {
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
-        title: const Text('Progress Quest - New Character'),
+        title: Text(L10n.of(context).newCharacterTitle),
         centerTitle: true,
       ),
       body: SingleChildScrollView(
@@ -231,7 +231,7 @@ class _NewCharacterScreenState extends State<NewCharacterScreen> {
             FilledButton.icon(
               onPressed: _onSold,
               icon: const Icon(Icons.check),
-              label: const Text('Sold!'),
+              label: Text(L10n.of(context).soldButton),
               style: FilledButton.styleFrom(
                 padding: const EdgeInsets.symmetric(vertical: 16),
               ),