fix(frontend): getUpcomingSchedules에 flattenScheduleResponse 적용

- 백엔드 응답(날짜별 그룹 객체)을 배열로 변환
- 필드명 snake_case 통일 (category_id, category_name, category_color)

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

frontend-temp/src/api/schedules.js

@@ -16,9 +16,9 @@ function flattenScheduleResponse(data) {
       schedules.push({
         ...schedule,
         date,
-        categoryId: category.id,
-        categoryName: category.name,
-        categoryColor: category.color,
+        category_id: category.id,
+        category_name: category.name,
+        category_color: category.color,
       });
     }
   }
@@ -40,7 +40,8 @@ export async function getSchedules(year, month) {
  */
 export async function getUpcomingSchedules(limit = 3) {
   const today = getTodayKST();
-  return fetchApi(`/schedules?startDate=${today}&limit=${limit}`);
+  const data = await fetchApi(`/schedules?startDate=${today}&limit=${limit}`);
+  return flattenScheduleResponse(data);
 }
 
 /**