feat(mobile): 일정 상세 페이지 액션바에 뒤로가기 버튼 추가

- ChevronLeft 아이콘 + history.back()
- 오른쪽 w-9 빈 영역으로 제목 중앙 정렬 유지

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

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

@@ -742,10 +742,14 @@ function MobileScheduleDetail() {
     <div className="mobile-layout-container bg-gray-50">
       {/* 헤더 */}
       <div className="flex-shrink-0 bg-white border-b border-gray-100">
-        <div className="flex items-center justify-center h-14 px-4">
-          <span className="text-base font-bold" style={{ color: schedule.category?.color }}>
+        <div className="flex items-center h-14 px-2">
+          <button onClick={() => window.history.back()} className="p-2 -ml-0.5 rounded-lg active:bg-gray-100 text-gray-600">
+            <ChevronLeft size={22} />
+          </button>
+          <span className="flex-1 text-center text-base font-bold" style={{ color: schedule.category?.color }}>
             {schedule.category?.name}
           </span>
+          <div className="w-9" />
         </div>
       </div>