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/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; }
+}