refactor: btn-game CSS 정리 및 LoginPage.css 삭제

- .btn-game에 display:inline-block, text-decoration:none 추가 → HomePage inline style 제거
- LoginPage.css 삭제 (deprecated, 임포트 없음)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/index.css

@@ -36,6 +36,7 @@ a:hover {
 
 .btn-game {
   position: relative;
+  display: inline-block;
   border: none;
   color: #fff;
   padding: 14px 40px;
@@ -45,6 +46,7 @@ a:hover {
   background: url('/images/btn_normal.webp') center/100% 100% no-repeat;
   letter-spacing: 0.05em;
   overflow: hidden;
+  text-decoration: none;
   transition: transform 0.15s, filter 0.15s;
 }
 .btn-game:hover { transform: translateY(-1px); filter: brightness(1.2); }