feat(admin): 관리자 대시보드와 미디어 자산 UI를 추가

src/entities/media/model/mockMediaManifest.ts

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+import { SCENE_THEMES } from '@/entities/scene';
+import { SOUND_PRESETS } from '@/entities/session';
+import type { MediaManifest } from './types';
+
+export const DEFAULT_MEDIA_MANIFEST: MediaManifest = {
+  version: 'local-fallback-v1',
+  updatedAt: '2026-03-09T00:00:00.000Z',
+  cdnBaseUrl: null,
+  scenes: SCENE_THEMES.map((scene) => ({
+    sceneId: scene.id,
+    cardUrl: scene.managedCardPhotoUrl ?? scene.cardPhotoUrl,
+    stageUrl: scene.managedCardPhotoUrl ?? scene.cardPhotoUrl,
+    mobileStageUrl: scene.managedCardPhotoUrl ?? scene.cardPhotoUrl,
+    hdStageUrl: scene.managedCardPhotoUrl ?? scene.cardPhotoUrl,
+    placeholderGradient: scene.previewGradient,
+  })),
+  sounds: SOUND_PRESETS.map((preset) => ({
+    presetId: preset.id,
+    previewUrl: null,
+    loopUrl: null,
+    fallbackLoopUrl: null,
+    mimeType: null,
+    durationSec: null,
+    defaultVolume: null,
+  })),
+};