feat(ui): 화면들에 레트로 UI 스타일 적용

- front_screen: 레트로 패널 및 버튼 스타일
- game_play_screen: 레트로 색상 및 초기 BGM 로직 개선
- mobile_carousel_layout: 레트로 테마 적용
- carousel_nav_bar: 골드 액센트 색상 적용

lib/src/features/game/widgets/carousel_nav_bar.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import 'package:askiineverdie/data/game_text_l10n.dart' as l10n;
+import 'package:askiineverdie/src/shared/retro_colors.dart';
 
 /// 캐로셀 페이지 인덱스
 enum CarouselPage {
@@ -13,7 +14,7 @@ enum CarouselPage {
   story, // 6: 스토리
 }
 
-/// 캐로셀 네비게이션 바
+/// 캐로셀 네비게이션 바 (레트로 스타일)
 ///
 /// 7개의 페이지 버튼을 표시하고 현재 페이지를 하이라이트.
 /// 버튼 탭 시 해당 페이지로 이동.
@@ -31,10 +32,12 @@ class CarouselNavBar extends StatelessWidget {
   Widget build(BuildContext context) {
     return Container(
       height: 56,
-      padding: const EdgeInsets.symmetric(horizontal: 4, vertical: 4),
-      decoration: BoxDecoration(
-        color: Theme.of(context).colorScheme.surfaceContainerHighest,
-        border: Border(top: BorderSide(color: Theme.of(context).dividerColor)),
+      padding: const EdgeInsets.symmetric(horizontal: 2, vertical: 4),
+      decoration: const BoxDecoration(
+        color: RetroColors.darkBrown,
+        border: Border(
+          top: BorderSide(color: RetroColors.gold, width: 2),
+        ),
       ),
       child: Row(
         children: CarouselPage.values.map((page) {
@@ -52,7 +55,7 @@ class CarouselNavBar extends StatelessWidget {
   }
 }
 
-/// 개별 네비게이션 버튼
+/// 개별 네비게이션 버튼 (레트로 스타일)
 class _NavButton extends StatelessWidget {
   const _NavButton({
     required this.page,
@@ -67,33 +70,32 @@ class _NavButton extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final (icon, label) = _getIconAndLabel(page);
-    final theme = Theme.of(context);
-    final color = isSelected
-        ? theme.colorScheme.primary
-        : theme.colorScheme.onSurfaceVariant;
+    final color = isSelected ? RetroColors.gold : RetroColors.textDisabled;
+    final bgColor = isSelected
+        ? RetroColors.panelBgLight
+        : Colors.transparent;
 
-    return InkWell(
+    return GestureDetector(
       onTap: onTap,
-      borderRadius: BorderRadius.circular(8),
       child: Container(
+        margin: const EdgeInsets.symmetric(horizontal: 1),
         padding: const EdgeInsets.symmetric(vertical: 4),
-        decoration: isSelected
-            ? BoxDecoration(
-                color: theme.colorScheme.primaryContainer.withValues(
-                  alpha: 0.5,
-                ),
-                borderRadius: BorderRadius.circular(8),
-              )
-            : null,
+        decoration: BoxDecoration(
+          color: bgColor,
+          border: isSelected
+              ? Border.all(color: RetroColors.gold, width: 1)
+              : null,
+        ),
         child: Column(
           mainAxisAlignment: MainAxisAlignment.center,
           children: [
-            Icon(icon, size: 20, color: color),
+            Icon(icon, size: 18, color: color),
             const SizedBox(height: 2),
             Text(
               label,
               style: TextStyle(
-                fontSize: 9,
+                fontFamily: 'PressStart2P',
+                fontSize: 5,
                 fontWeight: isSelected ? FontWeight.bold : FontWeight.normal,
                 color: color,
               ),