feat(game): 포션 시스템 및 UI 패널 추가

- 포션 시스템 구현 (PotionService, Potion 모델)
- 포션 인벤토리 패널 위젯
- 활성 버프 패널 위젯
- 장비 스탯 패널 위젯
- 스킬 시스템 확장
- 일본어 번역 추가
- 전투 이벤트/상태 모델 개선

lib/src/core/model/game_state.dart

@@ -5,6 +5,7 @@ import 'package:askiineverdie/src/core/model/combat_state.dart';
 import 'package:askiineverdie/src/core/model/equipment_item.dart';
 import 'package:askiineverdie/src/core/model/equipment_slot.dart';
 import 'package:askiineverdie/src/core/model/item_stats.dart';
+import 'package:askiineverdie/src/core/model/potion.dart';
 import 'package:askiineverdie/src/core/model/skill.dart';
 import 'package:askiineverdie/src/core/util/deterministic_random.dart';
 
@@ -23,6 +24,7 @@ class GameState {
     ProgressState? progress,
     QueueState? queue,
     SkillSystemState? skillSystem,
+    PotionInventory? potionInventory,
     this.deathInfo,
   }) : rng = DeterministicRandom.clone(rng),
        traits = traits ?? Traits.empty(),
@@ -32,7 +34,8 @@ class GameState {
        spellBook = spellBook ?? SpellBook.empty(),
        progress = progress ?? ProgressState.empty(),
        queue = queue ?? QueueState.empty(),
-       skillSystem = skillSystem ?? SkillSystemState.empty();
+       skillSystem = skillSystem ?? SkillSystemState.empty(),
+       potionInventory = potionInventory ?? const PotionInventory();
 
   factory GameState.withSeed({
     required int seed,
@@ -44,6 +47,7 @@ class GameState {
     ProgressState? progress,
     QueueState? queue,
     SkillSystemState? skillSystem,
+    PotionInventory? potionInventory,
     DeathInfo? deathInfo,
   }) {
     return GameState(
@@ -56,6 +60,7 @@ class GameState {
       progress: progress,
       queue: queue,
       skillSystem: skillSystem,
+      potionInventory: potionInventory,
       deathInfo: deathInfo,
     );
   }
@@ -72,6 +77,9 @@ class GameState {
   /// 스킬 시스템 상태 (Phase 3)
   final SkillSystemState skillSystem;
 
+  /// 물약 인벤토리
+  final PotionInventory potionInventory;
+
   /// 사망 정보 (Phase 4, null이면 생존 중)
   final DeathInfo? deathInfo;
 
@@ -88,6 +96,7 @@ class GameState {
     ProgressState? progress,
     QueueState? queue,
     SkillSystemState? skillSystem,
+    PotionInventory? potionInventory,
     DeathInfo? deathInfo,
     bool clearDeathInfo = false,
   }) {
@@ -101,6 +110,7 @@ class GameState {
       progress: progress ?? this.progress,
       queue: queue ?? this.queue,
       skillSystem: skillSystem ?? this.skillSystem,
+      potionInventory: potionInventory ?? this.potionInventory,
       deathInfo: clearDeathInfo ? null : (deathInfo ?? this.deathInfo),
     );
   }