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>

db_management/001_create_schema.sql

@@ -405,7 +405,7 @@ CREATE TABLE IF NOT EXISTS outputs (
   generated_by INT,                         -- FK는 later
   generated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
   version INT DEFAULT 1,
-  outputs_directory_path VARCHAR(500),      -- storage/.../B09_wf6_Estimation/v1/
+  outputs_directory_path VARCHAR(500),      -- storage/.../B09_Estimation/v1/
   metadata JSON                             -- template_used, company_name, project_name, total_cost, generation_time_sec
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
 
@@ -415,7 +415,7 @@ CREATE TABLE IF NOT EXISTS output_files (
   output_id INT NOT NULL,                   -- FK는 later
   file_type VARCHAR(50),                    -- xlsx, pdf, dxf, dwg, json, zip
   original_filename VARCHAR(255),
-  output_file_path VARCHAR(500),            -- storage/.../B09_wf6_Estimation/v1/...
+  output_file_path VARCHAR(500),            -- storage/.../B09_Estimation/v1/...
   file_size_mb FLOAT,
   created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
   download_count INT DEFAULT 0