feat(l10n): 엔딩 스크롤 속도업 안내 문자열 추가

- endingHoldToSpeedUp: 길게 누르면 빨리 스크롤
- 한/영/일/중 4개 언어 지원

lib/l10n/app_localizations.dart

@@ -646,6 +646,12 @@ abstract class L10n {
   /// In en, this message translates to:
   /// **'TAP TO SKIP'**
   String get endingTapToSkip;
+
+  /// Hold to speed up scrolling hint
+  ///
+  /// In en, this message translates to:
+  /// **'HOLD TO SPEED UP'**
+  String get endingHoldToSpeedUp;
 }
 
 class _L10nDelegate extends LocalizationsDelegate<L10n> {