보스 이미지 loading=lazy, decoding=async 추가

이전 revert 과정에서 함께 사라졌던 이미지 비동기 로딩 속성 복원.
애니메이션과 무관한 최적화이므로 유지.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/features/boss-crystal/pc/user/BossSelector.jsx

@@ -102,7 +102,7 @@ export default function BossSelector({ characterName, bosses, selections, onChan
                     className="shrink-0 w-11 h-11 rounded-lg overflow-hidden"
                     style={{ background: 'var(--surface-nested)' }}
                   >
-                    <img src={boss.image_url || '/default.png'} alt={boss.name} className="w-full h-full object-cover" />
+                    <img src={boss.image_url || '/default.png'} alt={boss.name} loading="lazy" decoding="async" className="w-full h-full object-cover" />
                   </div>
                   <span className="text-base font-medium leading-tight whitespace-nowrap overflow-hidden text-ellipsis">{boss.name}</span>
                 </div>

frontend/src/features/boss-crystal/pc/user/CharacterPanel.jsx

@@ -48,6 +48,8 @@ function CharacterContent({ char, selections, bosses }) {
               className="w-full h-full object-contain scale-[3] origin-center select-none"
               style={{ imageRendering: 'pixelated' }}
               draggable={false}
+              loading="lazy"
+              decoding="async"
             />
           ) : (
             <span className="text-4xl" style={{ color: 'var(--text-dim)' }}>?</span>
@@ -79,7 +81,7 @@ function CharacterContent({ char, selections, bosses }) {
                           borderColor: 'var(--panel-border)',
                         }}
                       >
-                        <img src={item.boss.image_url || '/default.png'} alt="" draggable={false} className="w-full h-full object-cover select-none" />
+                        <img src={item.boss.image_url || '/default.png'} alt="" draggable={false} loading="lazy" decoding="async" className="w-full h-full object-cover select-none" />
                       </div>
                       <div className="flex justify-center">
                         <div