refactor(responsive): Phase 3 코드 품질 개선

- Blog/BlogMarketing/Subscription/MusicStudio: 미사용 useIsMobile 제거
- Subscription: 미사용 Link import 제거
- Blog.css: 중복 display:block 제거
- BlogMarketing: dead prop onGenerate 제거
- Todo: 카드 버튼 터치 타겟 26→36px 확대

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

src/pages/blog-marketing/BlogMarketing.jsx

@@ -1,5 +1,4 @@
 import React, { useState, useEffect, useCallback, useRef } from 'react';
-import { useIsMobile } from '../../hooks/useIsMobile';
 import PullToRefresh from '../../components/PullToRefresh';
 import FAB from '../../components/FAB';
 import {
@@ -84,7 +83,6 @@ function usePollTask(onDone) {
 
 /* ══════════════════════════════════════════════════════════════════════════ */
 export default function BlogMarketing() {
-  const isMobile = useIsMobile();
   const [tab, setTab] = useState('dashboard');
   const [status, setStatus] = useState(null);
 
@@ -133,7 +131,7 @@ export default function BlogMarketing() {
       </nav>
 
       {tab === 'dashboard' && <DashboardTab />}
-      {tab === 'research' && <ResearchTab onGenerate={(id) => { setTab('write'); }} />}
+      {tab === 'research' && <ResearchTab />}
       {tab === 'write' && <WriteTab />}
       {tab === 'posts' && <PostsTab />}