feat(ebook): 풀이 진행상황 유지 + 처음부터 다시 풀기 + 복습 등록 복원

- 마운트 시 GET /study-logs 로 이미 등록된 문제 복원 (registeredProblems)
- selectedAnswers/revealed/pageIndex localStorage 영속화
- '처음부터 다시 풀기' 버튼 (복습 등록은 유지, 풀이만 초기화)
- 진행률 표시: 풀이 N/30 · 복습 등록 N/30
- title 형식에 문제 번호 포함 ([grade] #N question)

frontend/src/app/exams/sample-ebook/[id]/page.tsx

@@ -9,7 +9,7 @@ import AppShell from '@/components/layout/AppShell';
 import { Icon } from '@/components/ui/Icon';
 import { Badge, Button, Card } from '@/components/ui/primitives';
 import { theme } from '@/styles/theme';
-import { api, type Subject } from '@/lib/api';
+import { api, type Subject, type StudyLog } from '@/lib/api';
 import { EBOOK_MAP } from '../data/index';
 import type { SampleProblem } from '../data/types';
 
@@ -41,6 +41,7 @@ export default function SampleEbookDynamicPage() {
   return (
     <AppShell>
       <EbookViewer
+        id={id}
         title={ebook.title}
         grade={ebook.grade}
         problems={ebook.problems}
@@ -52,15 +53,39 @@ export default function SampleEbookDynamicPage() {
 // ─── 뷰어 ─────────────────────────────────────────────────────────────────────
 
 interface EbookViewerProps {
+  id: string;
   title: string;
   grade: string;
   problems: SampleProblem[];
 }
 
-function EbookViewer({ title, grade, problems }: EbookViewerProps) {
-  const [pageIndex, setPageIndex] = useState(0);
-  const [selectedAnswers, setSelectedAnswers] = useState<Record<number, number>>({});
-  const [revealed, setRevealed] = useState<Record<number, boolean>>({});
+function EbookViewer({ id, title, grade, problems }: EbookViewerProps) {
+  const storageKey = `reloop-ebook-${id}`;
+
+  const [pageIndex, setPageIndex] = useState<number>(() => {
+    if (typeof window === 'undefined') return 0;
+    try {
+      const saved = localStorage.getItem(`${storageKey}-page`);
+      return saved ? parseInt(saved, 10) : 0;
+    } catch { return 0; }
+  });
+
+  const [selectedAnswers, setSelectedAnswers] = useState<Record<number, number>>(() => {
+    if (typeof window === 'undefined') return {};
+    try {
+      const saved = localStorage.getItem(`${storageKey}-answers`);
+      return saved ? (JSON.parse(saved) as Record<number, number>) : {};
+    } catch { return {}; }
+  });
+
+  const [revealed, setRevealed] = useState<Record<number, boolean>>(() => {
+    if (typeof window === 'undefined') return {};
+    try {
+      const saved = localStorage.getItem(`${storageKey}-revealed`);
+      return saved ? (JSON.parse(saved) as Record<number, boolean>) : {};
+    } catch { return {}; }
+  });
+
   const [fading, setFading] = useState(false);
 
   // 자기 평가 관련 state
@@ -72,6 +97,19 @@ function EbookViewer({ title, grade, problems }: EbookViewerProps) {
   const total = problems.length;
   const problem = problems[pageIndex];
 
+  // localStorage 동기화
+  useEffect(() => {
+    localStorage.setItem(`${storageKey}-answers`, JSON.stringify(selectedAnswers));
+  }, [storageKey, selectedAnswers]);
+
+  useEffect(() => {
+    localStorage.setItem(`${storageKey}-revealed`, JSON.stringify(revealed));
+  }, [storageKey, revealed]);
+
+  useEffect(() => {
+    localStorage.setItem(`${storageKey}-page`, String(pageIndex));
+  }, [storageKey, pageIndex]);
+
   // 마운트 시 수학 subject ID 조회
   useEffect(() => {
     api.get<Subject[]>('/subjects').then((res) => {
@@ -82,6 +120,25 @@ function EbookViewer({ title, grade, problems }: EbookViewerProps) {
     });
   }, []);
 
+  // 마운트 시 기존 복습 등록 상태 복원
+  useEffect(() => {
+    const prefix = `[${grade}]`;
+    api.get<StudyLog[]>('/study-logs', { params: { limit: 200 } })
+      .then((res) => {
+        const registered = new Set<number>();
+        for (const log of res.data) {
+          if (log.title.startsWith(prefix)) {
+            const match = log.title.match(/^\[.+?\]\s*#(\d+)/);
+            if (match) registered.add(parseInt(match[1], 10));
+          }
+        }
+        setRegisteredProblems(registered);
+      })
+      .catch(() => {
+        // 실패해도 빈 Set 유지
+      });
+  }, [grade]);
+
   const goTo = useCallback(
     (index: number) => {
       if (index < 0 || index >= total || fading) return;
@@ -124,7 +181,7 @@ function EbookViewer({ title, grade, problems }: EbookViewerProps) {
     try {
       const res = await api.post<{ nextReview: { scheduledAt: string } }>('/study-logs', {
         subjectId: mathSubjectId,
-        title: `[${grade}] ${problem.question.slice(0, 50)}`,
+        title: `[${grade}] #${problem.number} ${problem.question.slice(0, 40)}`,
         difficulty: DIFFICULTY_SCORE[problem.difficulty],
         result: isCorrect ? 'correct' : 'incorrect',
         selfDifficulty: sd,
@@ -141,6 +198,17 @@ function EbookViewer({ title, grade, problems }: EbookViewerProps) {
     }
   }, [mathSubjectId, registeredProblems, registeringProblem, problem, grade, isCorrect, chosen]);
 
+  const handleReset = () => {
+    if (!window.confirm('모든 풀이 기록을 초기화할까? 복습 등록은 유지돼.')) return;
+    setSelectedAnswers({});
+    setRevealed({});
+    setPageIndex(0);
+    localStorage.removeItem(`${storageKey}-answers`);
+    localStorage.removeItem(`${storageKey}-revealed`);
+    localStorage.removeItem(`${storageKey}-page`);
+  };
+
+  const solvedCount = Object.keys(revealed).filter((k) => revealed[parseInt(k, 10)]).length;
   const alreadyRegistered = registeredProblems.has(problem.number);
   const scheduledDate = registeredDates[problem.number];
 
@@ -153,6 +221,15 @@ function EbookViewer({ title, grade, problems }: EbookViewerProps) {
           문제집으로 돌아가기
         </BackLink>
         <HeaderRight>
+          <ResetButton
+            type="button"
+            $variant="ghost"
+            $size="sm"
+            onClick={handleReset}
+          >
+            <Icon name="clock-counter-clockwise" size={13} />
+            처음부터 다시 풀기
+          </ResetButton>
           <PreviewBadge $variant="default">
             <Icon name="book-open-text" size={11} />
             샘플 미리보기
@@ -322,9 +399,14 @@ function EbookViewer({ title, grade, problems }: EbookViewerProps) {
           <Icon name="caret-left" size={16} />
           이전 페이지
         </NavButton>
-        <PageIndicator>
-          {pageIndex + 1} / {total}
-        </PageIndicator>
+        <PageNavCenter>
+          <PageIndicator>
+            {pageIndex + 1} / {total}
+          </PageIndicator>
+          <ProgressText>
+            풀이 {solvedCount}/{total} · 복습 등록 {registeredProblems.size}/{total}
+          </ProgressText>
+        </PageNavCenter>
         <NavButton
           type="button"
           $variant="ghost"
@@ -406,6 +488,16 @@ const BackLink = styled(Link)`
 const HeaderRight = styled.div`
   display: flex;
   align-items: center;
+  gap: ${theme.space.sm};
+`;
+
+const ResetButton = styled(Button)`
+  color: ${theme.color.textMute};
+  font-size: 12px;
+
+  &:hover {
+    color: ${theme.color.textSub};
+  }
 `;
 
 const PreviewBadge = styled(Badge)`
@@ -758,6 +850,13 @@ const NavButton = styled(Button)`
   min-width: 110px;
 `;
 
+const PageNavCenter = styled.div`
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 4px;
+`;
+
 const PageIndicator = styled.span`
   font-size: 15px;
   font-weight: 600;
@@ -766,6 +865,13 @@ const PageIndicator = styled.span`
   text-align: center;
 `;
 
+const ProgressText = styled.span`
+  font-size: 11px;
+  color: ${theme.color.textMute};
+  text-align: center;
+  white-space: nowrap;
+`;
+
 const InfoBox = styled(Card)`
   display: flex;
   align-items: flex-start;