feat(app): 테마 시스템 및 스플래시 화면 추가

- AppTheme 클래스 분리 (app_theme.dart)
- 스플래시 화면 추가 (splash_screen.dart)
- app.dart 경량화

lib/src/app_theme.dart

@@ -0,0 +1,137 @@
+import 'package:flutter/material.dart';
+
+import 'package:asciineverdie/src/shared/retro_colors.dart';
+
+/// 앱 테마 (Dark Fantasy 스타일)
+ThemeData buildAppTheme() => ThemeData(
+  colorScheme: RetroColors.darkColorScheme,
+  scaffoldBackgroundColor: RetroColors.deepBrown,
+  useMaterial3: true,
+  // 카드/다이얼로그 레트로 배경
+  cardColor: RetroColors.darkBrown,
+  dialogTheme: const DialogThemeData(
+    backgroundColor: Color(0xFF24283B),
+    titleTextStyle: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 15,
+      color: Color(0xFFE0AF68),
+    ),
+  ),
+  // 앱바 레트로 스타일
+  appBarTheme: const AppBarTheme(
+    backgroundColor: Color(0xFF24283B),
+    foregroundColor: Color(0xFFC0CAF5),
+    titleTextStyle: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 15,
+      color: Color(0xFFE0AF68),
+    ),
+  ),
+  // 버튼 테마 (inherit: false로 애니메이션 lerp 오류 방지)
+  filledButtonTheme: FilledButtonThemeData(
+    style: FilledButton.styleFrom(
+      backgroundColor: const Color(0xFF3D4260),
+      foregroundColor: const Color(0xFFC0CAF5),
+      textStyle: const TextStyle(
+        inherit: false,
+        fontFamily: 'PressStart2P',
+        fontSize: 14,
+        color: Color(0xFFC0CAF5),
+      ),
+    ),
+  ),
+  outlinedButtonTheme: OutlinedButtonThemeData(
+    style: OutlinedButton.styleFrom(
+      foregroundColor: const Color(0xFFE0AF68),
+      side: const BorderSide(color: Color(0xFFE0AF68), width: 2),
+      textStyle: const TextStyle(
+        inherit: false,
+        fontFamily: 'PressStart2P',
+        fontSize: 14,
+        color: Color(0xFFE0AF68),
+      ),
+    ),
+  ),
+  textButtonTheme: TextButtonThemeData(
+    style: TextButton.styleFrom(
+      foregroundColor: const Color(0xFFC0CAF5),
+      textStyle: const TextStyle(
+        inherit: false,
+        fontFamily: 'PressStart2P',
+        fontSize: 14,
+        color: Color(0xFFC0CAF5),
+      ),
+    ),
+  ),
+  // 텍스트 테마
+  textTheme: const TextTheme(
+    headlineLarge: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 20,
+      color: Color(0xFFE0AF68),
+    ),
+    headlineMedium: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 16,
+      color: Color(0xFFE0AF68),
+    ),
+    headlineSmall: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 15,
+      color: Color(0xFFE0AF68),
+    ),
+    titleLarge: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 15,
+      color: Color(0xFFC0CAF5),
+    ),
+    titleMedium: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 14,
+      color: Color(0xFFC0CAF5),
+    ),
+    titleSmall: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 14,
+      color: Color(0xFFC0CAF5),
+    ),
+    bodyLarge: TextStyle(fontSize: 18, color: Color(0xFFC0CAF5)),
+    bodyMedium: TextStyle(fontSize: 17, color: Color(0xFFC0CAF5)),
+    bodySmall: TextStyle(fontSize: 15, color: Color(0xFFC0CAF5)),
+    labelLarge: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 14,
+      color: Color(0xFFC0CAF5),
+    ),
+    labelMedium: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 14,
+      color: Color(0xFFC0CAF5),
+    ),
+    labelSmall: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 13,
+      color: Color(0xFFC0CAF5),
+    ),
+  ),
+  // 칩 테마
+  chipTheme: const ChipThemeData(
+    backgroundColor: Color(0xFF2A2E3F),
+    labelStyle: TextStyle(
+      fontFamily: 'PressStart2P',
+      fontSize: 14,
+      color: Color(0xFFC0CAF5),
+    ),
+    side: BorderSide(color: Color(0xFF545C7E)),
+  ),
+  // 리스트 타일 테마
+  listTileTheme: const ListTileThemeData(
+    textColor: Color(0xFFC0CAF5),
+    iconColor: Color(0xFFE0AF68),
+  ),
+  // 프로그레스 인디케이터
+  progressIndicatorTheme: const ProgressIndicatorThemeData(
+    color: Color(0xFFE0AF68),
+    linearTrackColor: Color(0xFF3B4261),
+  ),
+);