feat: 게임 시작/다운로드 버튼 통합

- 단일 "게임 시작" 버튼으로 통합
- 런처 설치 시 게임 실행, 미설치 시 자동 다운로드 (blur 감지)

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

src/components/DownloadSection.css

@@ -19,45 +19,29 @@
   margin: 0 0 28px;
 }
 
-.btn-download {
+.btn-play {
   display: inline-block;
-  padding: 16px 48px;
+  padding: 18px 64px;
   background: #BACDB0;
   color: #2E2C2F;
   border: none;
   border-radius: 8px;
-  font-size: 1.1rem;
+  font-size: 1.2rem;
   font-weight: 700;
-  text-decoration: none;
   cursor: pointer;
   transition: opacity 0.2s, transform 0.15s;
   letter-spacing: 0.05em;
 }
 
-.btn-download:hover {
+.btn-play:hover {
   opacity: 0.9;
   transform: translateY(-1px);
 }
 
-.btn-launch {
-  display: inline-block;
-  padding: 16px 48px;
-  background: transparent;
-  color: #BACDB0;
-  border: 2px solid #BACDB0;
-  border-radius: 8px;
-  font-size: 1.1rem;
-  font-weight: 700;
-  cursor: pointer;
-  transition: background 0.2s, color 0.2s, transform 0.15s;
-  letter-spacing: 0.05em;
-  margin-left: 16px;
-}
-
-.btn-launch:hover {
-  background: #BACDB0;
-  color: #2E2C2F;
-  transform: translateY(-1px);
+.btn-play:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+  transform: none;
 }
 
 .launch-hint {