feat(study): iPad 손글씨 학습 세션 frontend (Phase 1)

PR-2: 자격증/어학 학습 세션 UI. iPad Safari + Apple Pencil 지원.

신규 컴포넌트:
- HandwriteCanvas — perfect-freehand + PointerEvents (압력/tilt) +
  palm rejection (pointerType==='pen') + DPR + touch-action:none +
  stroke 단위 undo/redo + 5초 idle / 5 stroke 자동 저장 +
  localStorage 백업 + PNG snapshot export
- StudyMetaEditor — study_type(certification/language) 토글, 자격증/어학
  분기 메타 입력, 어학 metadata.reading/meaning/unit_type
- SourceTextPanel — 원문 + 어학 메타 read-only 표시
- AssetList — 연결된 audio/video/scan/handwriting 표시 + 재생 + 연결/해제

라우트:
- /study → /study/write 리다이렉트
- /study/write — 도메인 토글 + 빠른 시작 폼 + 세션 목록
- /study/write/[id] — 좌측 메타/원문/asset, 우측 캔버스 (md+ 분할,
  모바일 위/아래)

Layout/Sidebar:
- 상단 nav 에 "공부" 추가 (메모와 뉴스 사이)
- Sidebar 메모/Inbox 섹션에 GraduationCap 아이콘 항목 추가

기타:
- frontend/package.json: perfect-freehand ^1.2.3 (MIT)
- THIRD_PARTY_LICENSES.md 신규 — perfect-freehand MIT 고지

플랜: ~/.claude/plans/scalable-chasing-stonebraker.md (PR-2)
신규 파일 lint:tokens 회귀 0 (기존 잔존 130 그대로).

THIRD_PARTY_LICENSES.md

@@ -0,0 +1,34 @@
+# Third Party Licenses
+
+본 프로젝트는 다음 오픈소스를 사용합니다.
+
+## perfect-freehand
+
+- License: **MIT**
+- Repository: https://github.com/steveruizok/perfect-freehand
+- Used by: `frontend/src/lib/components/HandwriteCanvas.svelte` — Apple Pencil 압력/tilt
+  를 반영한 손글씨 stroke 렌더링.
+
+```
+MIT License
+
+Copyright (c) 2021 Stephen Ruiz Ltd
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+```