fix(a11y): 글로벌 prefers-reduced-motion 추가 + Blog 버튼 위치 수정

- App.css: 글로벌 reduced-motion 블록 (모든 animation/transition 비활성화)
- index.css: scroll-behavior: smooth → auto (reduced-motion)
- BlogMarketing.css: 스피너 reduced-motion 처리
- Blog.css: 플로팅 토글 버튼 bottom-nav 위로 재배치

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

src/App.css

@@ -492,3 +492,15 @@
         flex: none;
     }
 }
+
+/* ── Accessibility: Reduced Motion ──────────────────────────────────── */
+
+@media (prefers-reduced-motion: reduce) {
+    *,
+    *::before,
+    *::after {
+        animation-duration: 0.01ms !important;
+        animation-iteration-count: 1 !important;
+        transition-duration: 0.01ms !important;
+    }
+}

src/index.css

@@ -115,6 +115,10 @@ html {
     -webkit-text-size-adjust: 100%;
 }
 
+@media (prefers-reduced-motion: reduce) {
+    html { scroll-behavior: auto; }
+}
+
 body {
     height: 100%;
     overflow: hidden;

src/pages/blog-marketing/BlogMarketing.css

@@ -148,3 +148,7 @@
   .bm-write-actions { flex-direction: column; }
   .bm-post-card__actions { flex-wrap: wrap; }
 }
+
+@media (prefers-reduced-motion: reduce) {
+  .bm-spinner { animation: none; }
+}

src/pages/blog/Blog.css

@@ -81,7 +81,7 @@
     display: none;
     position: fixed;
     /* 사이드바 토글 버튼(top-left) 과 겹치지 않도록 오른쪽 하단 배치 */
-    bottom: 24px;
+    bottom: calc(var(--bottom-nav-h, 64px) + var(--safe-area-bottom, 0px) + 16px);
     right: 24px;
     top: auto;
     left: auto;