feat: 웹 클라이언트 비주얼 오버홀 — 게임 에셋 적용

- 홈페이지: 드래곤 히어로 배경(bg_loading), fixed 헤더, 로고, 마을 배경 다운로드 섹션
- 로그인/회원가입: 고딕 성문 배경(bg_login), 카드 프레임, 게임 버튼
- 공통: btn-game, card-game, divider-game CSS 클래스 추가
- 게임 에셋 PNG → WebP 변환 (6MB → 80~234KB)
- 관리자 페이지: 로고/divider 추가

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

src/index.css

@@ -30,4 +30,52 @@ a {
 
 a:hover {
   opacity: 0.85;
+}
+
+/* ── Game asset common classes ───────────────────── */
+
+.btn-game {
+  position: relative;
+  border: none;
+  color: #fff;
+  padding: 14px 40px;
+  font-size: 1.1rem;
+  font-weight: 700;
+  cursor: pointer;
+  background: url('/images/btn_normal.webp') center/100% 100% no-repeat;
+  letter-spacing: 0.05em;
+  overflow: hidden;
+  transition: transform 0.15s, brightness 0.15s;
+}
+.btn-game:hover { transform: translateY(-1px); filter: brightness(1.2); }
+.btn-game:disabled { opacity: 0.5; cursor: not-allowed; transform: none; }
+.btn-game > * { position: relative; z-index: 1; }
+
+.card-game {
+  position: relative;
+  background: rgba(0, 0, 0, 0.6);
+  border-radius: 12px;
+  overflow: hidden;
+}
+.card-game::before {
+  content: '';
+  position: absolute;
+  inset: -8px;
+  background: url('/images/card_frame.webp') center/100% 100% no-repeat;
+  pointer-events: none;
+  z-index: 0;
+}
+.card-game > * { position: relative; z-index: 1; }
+
+.divider-game {
+  width: 100%;
+  max-width: 400px;
+  height: 20px;
+  background: url('/images/divider.webp') center/contain no-repeat;
+  margin: 2rem auto;
+}
+
+@media (max-width: 768px) {
+  .card-game::before { display: none; }
+  .card-game { border: 1px solid rgba(186, 205, 176, 0.3); }
 }