feat(ui): 스피드 부스트 버튼 위젯 추가

- 5배속 버프 활성화 버튼
- 광고 시청으로 버프 획득
- 남은 시간 표시

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

@@ -0,0 +1,156 @@
+import 'package:flutter/material.dart';
+
+import 'package:asciineverdie/data/game_text_l10n.dart' as l10n;
+import 'package:asciineverdie/src/core/engine/iap_service.dart';
+import 'package:asciineverdie/src/shared/retro_colors.dart';
+
+/// 속도 부스트 버튼 위젯 (Phase 6)
+///
+/// 게임 화면에 표시되는 속도 부스트 활성화 버튼
+class SpeedBoostButton extends StatelessWidget {
+  const SpeedBoostButton({
+    super.key,
+    required this.isActive,
+    required this.remainingSeconds,
+    required this.onActivate,
+    this.boostMultiplier = 10,
+  });
+
+  /// 부스트 활성화 여부
+  final bool isActive;
+
+  /// 남은 시간 (초)
+  final int remainingSeconds;
+
+  /// 부스트 배율
+  final int boostMultiplier;
+
+  /// 활성화 콜백
+  final VoidCallback onActivate;
+
+  @override
+  Widget build(BuildContext context) {
+    final isPaidUser = IAPService.instance.isAdRemovalPurchased;
+    final gold = RetroColors.goldOf(context);
+    final goldDark = RetroColors.goldDarkOf(context);
+    final expColor = RetroColors.expOf(context);
+
+    // 부스트 활성화 중일 때
+    if (isActive) {
+      return _buildActiveButton(context, expColor);
+    }
+
+    // 부스트 비활성화 상태
+    return _buildInactiveButton(context, gold, goldDark, isPaidUser);
+  }
+
+  Widget _buildActiveButton(BuildContext context, Color expColor) {
+    return Container(
+      padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 8),
+      decoration: BoxDecoration(
+        color: expColor.withValues(alpha: 0.3),
+        borderRadius: BorderRadius.circular(8),
+        border: Border.all(color: expColor, width: 2),
+        boxShadow: [
+          BoxShadow(
+            color: expColor.withValues(alpha: 0.5),
+            blurRadius: 8,
+            spreadRadius: 1,
+          ),
+        ],
+      ),
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Text(
+                '⚡',
+                style: TextStyle(fontSize: 18, color: expColor),
+              ),
+              const SizedBox(width: 4),
+              Text(
+                '${boostMultiplier}x',
+                style: TextStyle(
+                  fontFamily: 'PressStart2P',
+                  fontSize: 14,
+                  color: expColor,
+                  fontWeight: FontWeight.bold,
+                ),
+              ),
+            ],
+          ),
+          const SizedBox(height: 4),
+          Text(
+            l10n.speedBoostRemaining(remainingSeconds),
+            style: TextStyle(
+              fontFamily: 'PressStart2P',
+              fontSize: 10,
+              color: expColor.withValues(alpha: 0.8),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+
+  Widget _buildInactiveButton(
+    BuildContext context,
+    Color gold,
+    Color goldDark,
+    bool isPaidUser,
+  ) {
+    return GestureDetector(
+      onTap: onActivate,
+      child: Container(
+        padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 8),
+        decoration: BoxDecoration(
+          color: gold.withValues(alpha: 0.2),
+          borderRadius: BorderRadius.circular(8),
+          border: Border.all(color: gold, width: 2),
+        ),
+        child: Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Text(
+              '⚡',
+              style: TextStyle(fontSize: 18, color: gold),
+            ),
+            const SizedBox(width: 4),
+            Text(
+              '${boostMultiplier}x',
+              style: TextStyle(
+                fontFamily: 'PressStart2P',
+                fontSize: 12,
+                color: gold,
+              ),
+            ),
+            // 광고 아이콘 (무료 유저만)
+            if (!isPaidUser) ...[
+              const SizedBox(width: 6),
+              Container(
+                padding: const EdgeInsets.symmetric(
+                  horizontal: 4,
+                  vertical: 2,
+                ),
+                decoration: BoxDecoration(
+                  color: Colors.white.withValues(alpha: 0.2),
+                  borderRadius: BorderRadius.circular(4),
+                ),
+                child: const Text(
+                  'AD',
+                  style: TextStyle(
+                    fontFamily: 'PressStart2P',
+                    fontSize: 8,
+                    color: Colors.white,
+                  ),
+                ),
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}