feat(arena): 아레나 화면 구현

- ArenaScreen: 아레나 메인 화면
- ArenaSetupScreen: 전투 설정 화면
- ArenaBattleScreen: 전투 진행 화면
- 관련 위젯 추가

lib/src/features/arena/arena_screen.dart

@@ -0,0 +1,168 @@
+import 'package:flutter/material.dart';
+
+import 'package:asciineverdie/src/core/model/hall_of_fame.dart';
+import 'package:asciineverdie/src/core/storage/hall_of_fame_storage.dart';
+import 'package:asciineverdie/src/features/arena/arena_setup_screen.dart';
+import 'package:asciineverdie/src/features/arena/widgets/arena_rank_card.dart';
+import 'package:asciineverdie/src/shared/retro_colors.dart';
+import 'package:asciineverdie/src/shared/widgets/retro_panel.dart';
+
+// 임시 문자열 (추후 l10n으로 이동)
+const _arenaTitle = 'LOCAL ARENA';
+const _arenaSubtitle = 'SELECT YOUR FIGHTER';
+const _arenaEmpty = 'Not enough heroes';
+const _arenaEmptyHint = 'Clear the game with 2+ characters';
+
+/// 로컬 아레나 메인 화면
+///
+/// 순위표 표시 및 도전하기 버튼
+class ArenaScreen extends StatefulWidget {
+  const ArenaScreen({super.key});
+
+  @override
+  State<ArenaScreen> createState() => _ArenaScreenState();
+}
+
+class _ArenaScreenState extends State<ArenaScreen> {
+  final HallOfFameStorage _storage = HallOfFameStorage();
+  HallOfFame? _hallOfFame;
+  bool _isLoading = true;
+
+  @override
+  void initState() {
+    super.initState();
+    _loadHallOfFame();
+  }
+
+  Future<void> _loadHallOfFame() async {
+    final hallOfFame = await _storage.load();
+
+    if (mounted) {
+      setState(() {
+        _hallOfFame = hallOfFame;
+        _isLoading = false;
+      });
+    }
+  }
+
+  /// 캐릭터 선택 시 바로 슬롯 선택 화면으로 이동
+  void _selectChallenger(HallOfFameEntry challenger) {
+    if (_hallOfFame == null || _hallOfFame!.count < 2) return;
+
+    Navigator.of(context).push(
+      MaterialPageRoute<void>(
+        builder: (context) => ArenaSetupScreen(
+          hallOfFame: _hallOfFame!,
+          initialChallenger: challenger,
+          onBattleComplete: _onBattleComplete,
+        ),
+      ),
+    );
+  }
+
+  void _onBattleComplete(HallOfFame updatedHallOfFame) {
+    setState(() {
+      _hallOfFame = updatedHallOfFame;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      backgroundColor: RetroColors.backgroundOf(context),
+      appBar: AppBar(
+        title: Text(
+          _arenaTitle,
+          style: const TextStyle(
+            fontFamily: 'PressStart2P',
+            fontSize: 12,
+          ),
+        ),
+        centerTitle: true,
+        backgroundColor: RetroColors.panelBgOf(context),
+      ),
+      body: SafeArea(
+        child: _isLoading
+            ? const Center(child: CircularProgressIndicator())
+            : _buildContent(),
+      ),
+    );
+  }
+
+  Widget _buildContent() {
+    final hallOfFame = _hallOfFame;
+    if (hallOfFame == null || hallOfFame.count < 2) {
+      return _buildEmptyState();
+    }
+
+    return Column(
+      children: [
+        // 순위표 (캐릭터 선택)
+        Expanded(child: _buildRankingList(hallOfFame)),
+      ],
+    );
+  }
+
+  Widget _buildEmptyState() {
+    return Center(
+      child: RetroPanel(
+        padding: const EdgeInsets.all(24),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(
+              Icons.sports_kabaddi,
+              size: 64,
+              color: RetroColors.textMutedOf(context),
+            ),
+            const SizedBox(height: 16),
+            Text(
+              _arenaEmpty,
+              style: TextStyle(
+                fontFamily: 'PressStart2P',
+                fontSize: 10,
+                color: RetroColors.textSecondaryOf(context),
+              ),
+            ),
+            const SizedBox(height: 8),
+            Text(
+              _arenaEmptyHint,
+              style: TextStyle(
+                fontFamily: 'PressStart2P',
+                fontSize: 7,
+                color: RetroColors.textMutedOf(context),
+              ),
+              textAlign: TextAlign.center,
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+
+  Widget _buildRankingList(HallOfFame hallOfFame) {
+    final rankedEntries = hallOfFame.rankedEntries;
+
+    return Padding(
+      padding: const EdgeInsets.all(12),
+      child: RetroGoldPanel(
+        title: _arenaSubtitle,
+        padding: const EdgeInsets.all(8),
+        child: ListView.builder(
+          itemCount: rankedEntries.length,
+          itemBuilder: (context, index) {
+            final entry = rankedEntries[index];
+            final score = HallOfFameArenaX.calculateArenaScore(entry);
+            return ArenaRankCard(
+              entry: entry,
+              rank: index + 1,
+              score: score,
+              onTap: () => _selectChallenger(entry),
+            );
+          },
+        ),
+      ),
+    );
+  }
+
+}