feat(youtube-tab): YoutubeTab 서브탭 shell 컴포넌트 + 스텁 탭 추가

src/pages/music/components/RevenueTab.jsx

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+export default function RevenueTab() {
+    return null;
+}

src/pages/music/components/TrendsTab.jsx

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+export default function TrendsTab() {
+    return null;
+}

src/pages/music/components/VideoProjectsTab.jsx

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+export default function VideoProjectsTab({ library, initialTrackId, onClearInitialTrack }) {
+    return null;
+}

src/pages/music/components/YoutubeTab.jsx

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+import { useState, useEffect } from 'react';
+import VideoProjectsTab from './VideoProjectsTab';
+import RevenueTab from './RevenueTab';
+import TrendsTab from './TrendsTab';
+
+export default function YoutubeTab({ library, initialTrackId, onClearInitialTrack }) {
+    const [subtab, setSubtab] = useState('video');
+
+    // initialTrackId가 들어오면 video 서브탭으로 전환
+    useEffect(() => {
+        if (initialTrackId) setSubtab('video');
+    }, [initialTrackId]);
+
+    return (
+        <div className="yt-container">
+            <nav className="yt-subtabs">
+                <button
+                    type="button"
+                    className={`yt-subtab ${subtab === 'video' ? 'is-active' : ''}`}
+                    onClick={() => setSubtab('video')}
+                >
+                    🎬 영상 제작
+                </button>
+                <button
+                    type="button"
+                    className={`yt-subtab ${subtab === 'revenue' ? 'is-active' : ''}`}
+                    onClick={() => setSubtab('revenue')}
+                >
+                    💰 수익 추적
+                </button>
+                <button
+                    type="button"
+                    className={`yt-subtab ${subtab === 'trends' ? 'is-active' : ''}`}
+                    onClick={() => setSubtab('trends')}
+                >
+                    📊 시장 트렌드
+                </button>
+            </nav>
+
+            {subtab === 'video' && (
+                <VideoProjectsTab
+                    library={library}
+                    initialTrackId={initialTrackId}
+                    onClearInitialTrack={onClearInitialTrack}
+                />
+            )}
+            {subtab === 'revenue' && <RevenueTab />}
+            {subtab === 'trends' && <TrendsTab />}
+        </div>
+    );
+}