보스 폼 사용성 개선

- 가격 입력 시 Tab으로 다음 활성 가격 칸으로 이동 (체크박스 건너뜀)
- 최대 인원 기본값 6 → 3 (요즘 보스 기본)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/Checkbox.jsx

@@ -2,7 +2,7 @@
  * 커스텀 체크박스
  * <Checkbox checked={x} onChange={(checked) => ...} />
  */
-export default function Checkbox({ checked, onChange, disabled, className = '', size = 'md' }) {
+export default function Checkbox({ checked, onChange, disabled, className = '', size = 'md', tabIndex }) {
   const sizeCls = size === 'sm' ? 'w-4 h-4' : 'w-5 h-5'
   const iconSize = size === 'sm' ? 'text-[10px]' : 'text-xs'
 
@@ -12,6 +12,7 @@ export default function Checkbox({ checked, onChange, disabled, className = '',
       role="checkbox"
       aria-checked={checked}
       disabled={disabled}
+      tabIndex={tabIndex}
       onClick={(e) => { e.stopPropagation(); !disabled && onChange?.(!checked) }}
       className={`${sizeCls} shrink-0 rounded-md border-2 flex items-center justify-center transition ${
         checked

frontend/src/features/boss-crystal/admin/BossForm.jsx

@@ -42,7 +42,7 @@ export default function BossForm() {
   const fileInputRef = useRef(null)
 
   const [name, setName] = useState('')
-  const [maxPartySize, setMaxPartySize] = useState(6)
+  const [maxPartySize, setMaxPartySize] = useState(3)
   const [imageFile, setImageFile] = useState(null)
   const [imagePreview, setImagePreview] = useState(null)
   const [existingImageUrl, setExistingImageUrl] = useState(null)
@@ -60,7 +60,7 @@ export default function BossForm() {
   useEffect(() => {
     if (!isEdit) {
       setName('')
-      setMaxPartySize(6)
+      setMaxPartySize(3)
       setImageFile(null)
       setImagePreview(null)
       setExistingImageUrl(null)
@@ -69,7 +69,7 @@ export default function BossForm() {
     }
     if (bossData) {
       setName(bossData.name || '')
-      setMaxPartySize(bossData.max_party_size || 6)
+      setMaxPartySize(bossData.max_party_size || 3)
       setExistingImageUrl(bossData.image_url || null)
       setImagePreview(null)
       setImageFile(null)
@@ -259,6 +259,7 @@ export default function BossForm() {
                       <Checkbox
                         checked={v.enabled}
                         onChange={(checked) => updateDifficulty(d.key, { enabled: checked })}
+                        tabIndex={-1}
                       />
                       <img
                         src={getDifficultyImageUrl(d.key)}