feat: 예능 썸네일 없을 때 카테고리 색상 기본 이미지 표시

- 썸네일 없으면 카테고리 색상 배경 + Tv 아이콘 표시
- PC/모바일 모두 적용
- 항상 flex 레이아웃 유지 (썸네일 유무와 무관)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/mobile/schedule/ScheduleDetail.jsx

@@ -487,17 +487,24 @@ function MobileVarietySection({ schedule }) {
 
   return (
     <div className="bg-white rounded-xl overflow-hidden shadow-sm">
-      <div className={`${hasThumbnail ? 'flex' : ''}`}>
+      <div className="flex">
         {/* 썸네일 */}
-        {hasThumbnail && (
-          <div className="flex-shrink-0 w-32">
+        <div className="flex-shrink-0 w-28">
+          {hasThumbnail ? (
             <img
               src={schedule.thumbnailUrl}
               alt={schedule.title}
               className="w-full h-full object-cover"
             />
-          </div>
-        )}
+          ) : (
+            <div
+              className="w-full h-full flex items-center justify-center"
+              style={{ backgroundColor: `${schedule.category?.color || '#06b6d4'}15` }}
+            >
+              <Tv size={32} style={{ color: schedule.category?.color || '#06b6d4' }} strokeWidth={1.5} />
+            </div>
+          )}
+        </div>
 
         {/* 콘텐츠 */}
         <div className="flex-1 p-4 flex flex-col justify-between">

frontend/src/pages/pc/public/schedule/sections/VarietySection.jsx

@@ -13,17 +13,24 @@ function VarietySection({ schedule }) {
 
   return (
     <div className="bg-white rounded-2xl overflow-hidden shadow-sm border border-gray-100">
-      <div className={`${hasThumbnail ? 'flex' : ''}`}>
+      <div className="flex">
         {/* 왼쪽: 썸네일 */}
-        {hasThumbnail && (
-          <div className="flex-shrink-0 w-44">
+        <div className="flex-shrink-0 w-44">
+          {hasThumbnail ? (
             <img
               src={schedule.thumbnailUrl}
               alt={schedule.title}
               className="w-full h-full object-cover"
             />
-          </div>
-        )}
+          ) : (
+            <div
+              className="w-full h-full flex items-center justify-center"
+              style={{ backgroundColor: `${schedule.category?.color || '#06b6d4'}15` }}
+            >
+              <Tv size={40} style={{ color: schedule.category?.color || '#06b6d4' }} strokeWidth={1.5} />
+            </div>
+          )}
+        </div>
 
         {/* 오른쪽: 콘텐츠 */}
         <div className="flex-1 p-5 flex flex-col justify-between">