feat(B07,B09): B07_Quantity 셸 페이지 신설 + B09_Estimation 접두사 정리 + 단계 순서 반영

- B09_wf6_Estimation -> B09_Estimation (폴더·파일·라우트 키/슬러그)
- B07_Quantity_UI_Page.ts 신설: 워크플로우 셸 + 좌측 [확정] 버튼
  (renderPendingWorkflow에 leftPanel 옵션 추가로 공용 셸 재사용)
- B07_Quantity_Router.py 신설: POST /api/projects/{id}/quantity/confirm
  -> complete_stage(4) 전이만 수행(본문 미구현), main.py 등록
- STAGE_KEYS 재정의: FILE_INPUT/PREPROCESS/PROFILE/SECTION/QUANTITY/DESIGN_DETAIL/ESTIMATION
- 스텝바 라벨·아이콘 4↔5 스왑(수량산출이 4차, 상세설계가 5차), A02 소개 문구 스왑
- 라우터 테이블에 b07-quantity 등록(플레이스홀더 제거), locale 키 5종 추가

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

main.py

@@ -40,6 +40,7 @@ from B06_Section.B06_Section_Router import router as b06_section_router
 from B06_Section.B06_Section_Router_Confirm import (
     router as b06_section_confirm_router,
 )
+from B07_Quantity.B07_Quantity_Router import router as b07_quantity_router
 from B08_DesignDetail.B08_DesignDetail_Router import router as b08_design_router
 from common_util.common_util_auth import require_company, verify_session
 from common_util.common_util_resource_monitor import sample_resources_loop
@@ -358,6 +359,7 @@ app.include_router(tiles_router, dependencies=protected_with_company)
 app.include_router(b05_route_router, dependencies=protected_with_company)
 app.include_router(b06_section_router, dependencies=protected_with_company)
 app.include_router(b06_section_confirm_router, dependencies=protected_with_company)
+app.include_router(b07_quantity_router, dependencies=protected_with_company)
 app.include_router(b08_design_router, dependencies=protected_with_company)