style(admin): 콘서트/예능 수정 폼 너비를 추가 폼과 통일

수정 폼이 화면 전체 너비를 쓰던 문제를 max-w-4xl 컨테이너로 감싸 추가 페이지와 동일하게 맞춤.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/pc/admin/schedules/edit/ConcertEditForm.jsx

@@ -241,7 +241,7 @@ function ConcertEditForm() {
         animate={{ opacity: 1, y: 0 }}
         transition={{ duration: 0.4, ease: [0.25, 0.1, 0.25, 1] }}
         onSubmit={handleSubmit}
-        className="space-y-6"
+        className="space-y-6 max-w-4xl mx-auto px-6 py-8"
       >
         <ConcertInfoSection
           title={title}

frontend/src/pages/pc/admin/schedules/edit/VarietyEditForm.jsx

@@ -128,7 +128,7 @@ function VarietyEditForm() {
         animate={{ opacity: 1, y: 0 }}
         transition={{ duration: 0.4 }}
         onSubmit={handleSubmit}
-        className="space-y-6"
+        className="space-y-6 max-w-4xl mx-auto px-6 py-8"
       >
         {/* 프로그램 정보 */}
         <div className="bg-white rounded-2xl border border-gray-100 shadow-sm p-6">