fix: dashboard 빈 화면 수정 - purgeNonMath 삭제 + API 에러 방어

- purgeNonMath 호출 삭제 (데이터 삭제 위험)
- Promise.all 내 API 호출에 .catch() 추가하여
  하나의 API 실패가 전체 대시보드를 깨지 않도록 방어

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

frontend/src/app/dashboard/page.tsx

@@ -95,15 +95,6 @@ function DashboardBody() {
     setCoachDismissed(window.localStorage.getItem(COACH_DISMISS_KEY) === '1');
   }, []);
 
-  useEffect(() => {
-    const purged = localStorage.getItem('reloop-purged-non-math');
-    if (!purged) {
-      api.post('/study-logs/purge-non-math').then(() => {
-        localStorage.setItem('reloop-purged-non-math', 'true');
-      }).catch(() => {});
-    }
-  }, []);
-
   useEffect(() => {
     let cancelled = false;
 
@@ -115,12 +106,12 @@ function DashboardBody() {
         const [meRes, summaryRes, recentRes, activityRes, subjectsRes, orgsRes, classesRes, assignmentsRes, calendarRes] = await Promise.all([
           api.get<MeUser>('/auth/me'),
           api.get<DashboardSummary>('/dashboard/summary'),
-          api.get<StudyLog[]>('/study-logs', { params: { limit: 5 } }),
+          api.get<StudyLog[]>('/study-logs', { params: { limit: 5 } }).catch(() => ({ data: [] as StudyLog[] })),
-          api.get<StudyLog[]>('/study-logs', { params: { limit: 200 } }),
+          api.get<StudyLog[]>('/study-logs', { params: { limit: 200 } }).catch(() => ({ data: [] as StudyLog[] })),
-          api.get<SubjectStats[]>('/stats/subjects'),
+          api.get<SubjectStats[]>('/stats/subjects').catch(() => ({ data: [] as SubjectStats[] })),
-          getMyOrganizations(),
+          getMyOrganizations().catch(() => [] as Organization[]),
-          getMyClasses(),
+          getMyClasses().catch(() => [] as Awaited<ReturnType<typeof getMyClasses>>),
-          getAssignments(),
+          getAssignments().catch(() => [] as Awaited<ReturnType<typeof getAssignments>>),
           getReviewCalendar(now.getFullYear(), now.getMonth() + 1).catch(() => ({ days: [] as CalendarDay[] })),
         ]);