feat: About 다이얼로그 추가, UI 최적화 및 서버 캐싱 강화

- About 다이얼로그 추가 (개발자 정보 및 개인정보처리방침)
- Markdown 렌더러 구현 (Bold, Italic, Code, Blockquote 지원)
- 전투 화면 하단 'About' 및 방문자 카운팅 UI 재배치 및 디자인 통일
- 프로덕션 환경에서 정적 파일 강력 캐싱 설정 (7일 유지)
- 파비콘 404 오류 해결을 위한 이모지 데이터 URI 추가
- 모바일 전투 화면 레이아웃 최적화 및 승리 연출 개선
- 일일 운영 지표(Daily Metrics) 수집 API 및 로직 추가

index.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Arena Picker</title>
+    <link rel="icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>⚔️</text></svg>">
     <style>
       html.app-booting,
       html.app-booting body {
@@ -63,9 +64,21 @@
             <span data-status-text>옵션 대기 중</span>
           </div>
         </div>
-        <p id="visitor-count" class="visitor-count" aria-live="polite">
-          방문자 확인 중
-        </p>
+        <div class="arena-meta">
+          <p id="visitor-count" class="visitor-count" aria-live="polite">
+            방문자 확인 중
+          </p>
+          <button
+            id="about-button"
+            class="about-button"
+            type="button"
+            aria-haspopup="dialog"
+            aria-controls="about-dialog"
+            aria-expanded="false"
+          >
+            About
+          </button>
+        </div>
       </section>
 
       <section class="intro-stage" aria-label="Arena 시작 화면">
@@ -207,6 +220,96 @@ Player 10</textarea
           </div>
         </form>
       </aside>
+
+      <div id="about-dialog" class="about-backdrop" hidden>
+        <section
+          class="about-dialog"
+          data-about-dialog
+          role="dialog"
+          aria-modal="true"
+          aria-labelledby="about-dialog-title"
+          tabindex="-1"
+        >
+          <header class="about-header">
+            <div>
+              <p class="eyebrow">About</p>
+              <h2 id="about-dialog-title">개발자정보</h2>
+            </div>
+            <button
+              class="about-close"
+              data-about-close
+              type="button"
+              aria-label="About 닫기"
+            >
+              X
+            </button>
+          </header>
+
+          <div class="about-tabs" role="tablist" aria-label="About 메뉴">
+            <button
+              id="about-tab-developer"
+              class="about-tab"
+              data-about-tab="developer"
+              type="button"
+              role="tab"
+              aria-selected="true"
+              aria-controls="about-panel-developer"
+            >
+              개발자정보
+            </button>
+            <button
+              id="about-tab-privacy"
+              class="about-tab"
+              data-about-tab="privacy"
+              type="button"
+              role="tab"
+              aria-selected="false"
+              aria-controls="about-panel-privacy"
+              tabindex="-1"
+            >
+              개인정보처리방침
+            </button>
+          </div>
+
+          <section
+            id="about-panel-developer"
+            class="about-panel"
+            data-about-panel="developer"
+            role="tabpanel"
+            aria-labelledby="about-tab-developer"
+          >
+            <dl class="about-fields">
+              <div class="about-field-row">
+                <dt>alias</dt>
+                <dd data-about-field="alias">horoli</dd>
+              </div>
+              <div class="about-field-row">
+                <dt>email</dt>
+                <dd data-about-field="email">sunha321@gmail.com</dd>
+              </div>
+              <div class="about-field-row">
+                <dt>github</dt>
+                <dd data-about-field="github">
+                  <a href="https://github.com/Horoli" target="_blank" rel="noreferrer">
+                    https://github.com/Horoli
+                  </a>
+                </dd>
+              </div>
+            </dl>
+          </section>
+
+          <section
+            id="about-panel-privacy"
+            class="about-panel"
+            data-about-panel="privacy"
+            role="tabpanel"
+            aria-labelledby="about-tab-privacy"
+            hidden
+          >
+            <div id="privacy-policy-content" class="about-markdown"></div>
+          </section>
+        </section>
+      </div>
     </main>
     <script type="module" src="/src/main.js"></script>
   </body>