refactor(model): SpellBook을 SkillBook으로 리네이밍

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

lib/src/features/new_character/new_character_screen.dart

@@ -261,7 +261,7 @@ class _NewCharacterScreenState extends State<NewCharacterScreen> {
       stats: finalStats,
       inventory: const Inventory(gold: 0, items: []),
       equipment: Equipment.empty(),
-      spellBook: SpellBook.empty(),
+      skillBook: SkillBook.empty(),
       progress: ProgressState.empty(),
       queue: QueueState.empty(),
     );