refactor(model): SpellBook을 SkillBook으로 리네이밍

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

lib/src/core/model/save_data.dart

@@ -13,7 +13,7 @@ class GameSave {
     required this.stats,
     required this.inventory,
     required this.equipment,
-    required this.spellBook,
+    required this.skillBook,
     required this.progress,
     required this.queue,
   });
@@ -26,7 +26,7 @@ class GameSave {
       stats: state.stats,
       inventory: state.inventory,
       equipment: state.equipment,
-      spellBook: state.spellBook,
+      skillBook: state.skillBook,
       progress: state.progress,
       queue: state.queue,
     );
@@ -38,7 +38,7 @@ class GameSave {
   final Stats stats;
   final Inventory inventory;
   final Equipment equipment;
-  final SpellBook spellBook;
+  final SkillBook skillBook;
   final ProgressState progress;
   final QueueState queue;
 
@@ -88,7 +88,7 @@ class GameSave {
         'sollerets': equipment.sollerets,
         'bestIndex': equipment.bestIndex,
       },
-      'spells': spellBook.spells
+      'skills': skillBook.skills
           .map((e) => {'name': e.name, 'rank': e.rank})
           .toList(),
       'progress': {
@@ -140,7 +140,7 @@ class GameSave {
     final equipmentJson = json['equipment'] as Map<String, dynamic>;
     final progressJson = json['progress'] as Map<String, dynamic>;
     final queueJson = (json['queue'] as List<dynamic>? ?? []).cast<dynamic>();
-    final spellsJson = (json['spells'] as List<dynamic>? ?? []).cast<dynamic>();
+    final skillsJson = (json['skills'] as List<dynamic>? ?? []).cast<dynamic>();
 
     return GameSave(
       version: json['version'] as int? ?? kSaveVersion,
@@ -192,10 +192,10 @@ class GameSave {
         sollerets: equipmentJson['sollerets'] as String? ?? '',
         bestIndex: equipmentJson['bestIndex'] as int? ?? 0,
       ),
-      spellBook: SpellBook(
-        spells: spellsJson
+      skillBook: SkillBook(
+        skills: skillsJson
             .map(
-              (e) => SpellEntry(
+              (e) => SkillEntry(
                 name: (e as Map<String, dynamic>)['name'] as String? ?? '',
                 rank: (e)['rank'] as String? ?? 'I',
               ),
@@ -261,7 +261,7 @@ class GameSave {
       stats: stats,
       inventory: inventory,
       equipment: equipment,
-      spellBook: spellBook,
+      skillBook: skillBook,
       progress: progress,
       queue: queue,
     );