feat(l10n): 국제화(L10n) 시스템 도입 및 하드코딩 텍스트 변환

- flutter_localizations 및 intl 패키지 추가
- l10n.yaml 설정 파일 및 app_ko.arb 메시지 파일 생성
- 모든 화면(app, front, game_play, new_character, save_picker)의 하드코딩 텍스트를 L10n 키로 변환
- 테스트 파일에 localizationsDelegates 추가하여 L10n 지원

test/features/new_character_screen_test.dart

@@ -1,12 +1,22 @@
+import 'package:askiineverdie/l10n/app_localizations.dart';
 import 'package:askiineverdie/src/core/model/game_state.dart';
 import 'package:askiineverdie/src/features/new_character/new_character_screen.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+/// 테스트용 MaterialApp 래퍼 (localization 포함)
+Widget _buildTestApp(Widget child) {
+  return MaterialApp(
+    localizationsDelegates: L10n.localizationsDelegates,
+    supportedLocales: L10n.supportedLocales,
+    home: child,
+  );
+}
+
 void main() {
   testWidgets('NewCharacterScreen renders main sections', (tester) async {
     await tester.pumpWidget(
-      MaterialApp(home: NewCharacterScreen(onCharacterCreated: (_) {})),
+      _buildTestApp(NewCharacterScreen(onCharacterCreated: (_) {})),
     );
 
     // 화면 타이틀 확인
@@ -29,7 +39,7 @@ void main() {
 
   testWidgets('Unroll button exists and can be tapped', (tester) async {
     await tester.pumpWidget(
-      MaterialApp(home: NewCharacterScreen(onCharacterCreated: (_) {})),
+      _buildTestApp(NewCharacterScreen(onCharacterCreated: (_) {})),
     );
 
     // Unroll 버튼 확인
@@ -50,8 +60,8 @@ void main() {
     GameState? createdState;
 
     await tester.pumpWidget(
-      MaterialApp(
-        home: NewCharacterScreen(
+      _buildTestApp(
+        NewCharacterScreen(
           onCharacterCreated: (state) {
             createdState = state;
           },
@@ -81,7 +91,7 @@ void main() {
 
   testWidgets('Stats section displays all six stats', (tester) async {
     await tester.pumpWidget(
-      MaterialApp(home: NewCharacterScreen(onCharacterCreated: (_) {})),
+      _buildTestApp(NewCharacterScreen(onCharacterCreated: (_) {})),
     );
 
     // 능력치 라벨들이 표시되는지 확인
@@ -98,7 +108,7 @@ void main() {
 
   testWidgets('Name text field exists', (tester) async {
     await tester.pumpWidget(
-      MaterialApp(home: NewCharacterScreen(onCharacterCreated: (_) {})),
+      _buildTestApp(NewCharacterScreen(onCharacterCreated: (_) {})),
     );
 
     // TextField 확인 (이름 입력 필드)