fix: YouTubeEditForm 일정 없을 때 에러 UI 추가

- 존재하지 않는 일정 ID 접근 시 빈 화면 대신 에러 UI 표시
- 이전 페이지/일정 목록 이동 버튼 제공

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend-temp/src/pages/pc/admin/schedules/edit/YouTubeEditForm.jsx

@@ -11,6 +11,8 @@ import {
   ChevronRight,
   Users,
   Check,
+  ArrowLeft,
+  Calendar,
 } from "lucide-react";
 import AdminLayout from "@/components/pc/admin/layout/Layout";
 import Toast from "@/components/common/Toast";
@@ -55,7 +57,7 @@ function YouTubeEditForm() {
   const [isInitialized, setIsInitialized] = useState(false);
 
   // 일정 데이터 로드
-  const { data: schedule, isLoading: scheduleLoading } = useQuery({
+  const { data: schedule, isLoading: scheduleLoading, isError: scheduleError } = useQuery({
     queryKey: ["schedule", id],
     queryFn: async () => {
       const token = localStorage.getItem("adminToken");
@@ -66,6 +68,7 @@ function YouTubeEditForm() {
       return res.json();
     },
     enabled: isAuthenticated && !!id,
+    retry: false,
   });
 
   // 멤버 목록 로드
@@ -173,8 +176,59 @@ function YouTubeEditForm() {
     );
   }
 
-  if (!schedule) {
-    return null;
+  if (scheduleError || !schedule) {
+    return (
+      <AdminLayout user={user}>
+        <div className="flex-1 flex items-center justify-center min-h-[calc(100vh-200px)]">
+          <div className="text-center px-6">
+            <motion.div
+              initial={{ opacity: 0, scale: 0.5 }}
+              animate={{ opacity: 1, scale: 1 }}
+              transition={{ duration: 0.5, type: "spring", stiffness: 100 }}
+              className="mb-8"
+            >
+              <div className="w-32 h-32 mx-auto bg-gradient-to-br from-primary/10 to-primary/5 rounded-3xl flex items-center justify-center">
+                <Calendar size={64} className="text-primary/40" />
+              </div>
+            </motion.div>
+            <motion.div
+              initial={{ opacity: 0, y: 20 }}
+              animate={{ opacity: 1, y: 0 }}
+              transition={{ delay: 0.2, duration: 0.5 }}
+              className="mb-8"
+            >
+              <h2 className="text-2xl font-bold text-gray-800 mb-3">
+                일정을 찾을 수 없습니다
+              </h2>
+              <p className="text-gray-500 leading-relaxed">
+                요청하신 일정이 존재하지 않거나 삭제되었을 수 있습니다.
+              </p>
+            </motion.div>
+            <motion.div
+              initial={{ opacity: 0, y: 20 }}
+              animate={{ opacity: 1, y: 0 }}
+              transition={{ delay: 0.4, duration: 0.5 }}
+              className="flex justify-center gap-4"
+            >
+              <button
+                onClick={() => window.history.back()}
+                className="flex items-center gap-2 px-6 py-3 border-2 border-primary text-primary rounded-xl font-medium hover:bg-primary hover:text-white transition-colors duration-200"
+              >
+                <ArrowLeft size={18} />
+                이전 페이지
+              </button>
+              <Link
+                to="/admin/schedule"
+                className="flex items-center gap-2 px-6 py-3 bg-gradient-to-r from-primary to-primary-dark text-white rounded-xl font-medium hover:shadow-lg hover:shadow-primary/30 transition-all duration-200"
+              >
+                <Calendar size={18} />
+                일정 목록
+              </Link>
+            </motion.div>
+          </div>
+        </div>
+      </AdminLayout>
+    );
   }
 
   const videoUrl = videoType === "shorts"