refactor(model): SpellBook을 SkillBook으로 리네이밍

- 게임 컨셉에 맞게 주문서 → 스킬북 용어 통일
- 관련 모든 참조 일괄 변경

lib/src/features/arena/arena_battle_screen.dart

@@ -172,7 +172,7 @@ class _ArenaBattleScreenState extends State<ArenaBattleScreen>
           message: '${widget.match.challenger.characterName} uses '
               '${turn.challengerSkillUsed}!',
           timestamp: DateTime.now(),
-          type: CombatLogType.spell,
+          type: CombatLogType.skill,
         ));
       }
 
@@ -223,7 +223,7 @@ class _ArenaBattleScreenState extends State<ArenaBattleScreen>
           message: '${widget.match.opponent.characterName} uses '
               '${turn.opponentSkillUsed}!',
           timestamp: DateTime.now(),
-          type: CombatLogType.spell,
+          type: CombatLogType.skill,
         ));
       }