feat(phase10C): 온보딩 + 프론트 재구성 — 수학 수능 4단원 + PS 트랙

- lib/api.ts: GoalType('math-suneung'|'ps'|...), MathUnit, MATH_UNIT_LABEL,
  MeUser{focusUnits,bojHandle}, ps.* helper (search/bookmark/sync/getProblem)
- onboarding/page.tsx: 5개 GoalType 옵션, math-suneung 단원 멀티셀렉트,
  ps 인풋 + 즉시 동기화 토글, focusUnits/bojHandle PATCH
- profile/page.tsx: 동일 컨트롤 노출 + 검증
- ps/{page,bookmarks/page,sync/page}.tsx: 검색·필터·북마크·동기화 신규 섹션
- components/ps/PsTabs.tsx, layout/{SideNav,BottomNav}.tsx, ui/Icon.tsx:
  PS 진입점 + Phosphor code/bookmark-simple 아이콘
- exam/shared.ts: getExamDurationSeconds → 수학 100분 고정
- 비수학 하드코딩 전면 제거: dashboard/exams/study/subjects/school/assignment
- 카피 갱신: landing/pricing/layout metadata + README + docs(user/admin/pipeline)
- .eslintrc.json: Next core lint 결정성 확보

Phase 10C DoD 통과 — pnpm build + lint 0 에러, /ps /ps/bookmarks /ps/sync 라우트 생성,
grep -rE '국어|영어|한국사|생활과 윤리|사회탐구' frontend/src docs = 0

frontend/src/app/exams/page.tsx

@@ -11,7 +11,7 @@ import { api, type ProblemSetSummary } from '@/lib/api';
 import { theme } from '@/styles/theme';
 
 const YEAR_FILTERS = [2026, 2025] as const;
-const SUBJECT_FILTERS = ['전체', '국어', '수학', '영어', '한국사', '생활과 윤리'] as const;
+const SUBJECT_FILTERS = ['전체', '수학'] as const;
 
 type YearFilter = number | 'all';
 type SubjectFilter = (typeof SUBJECT_FILTERS)[number];