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;
+    }
+}