fix: [brochure] v5 PPTX 빈 파일 수정 - body 그래디언트 → 단색 변경

- html2pptx가 CSS gradient 검출 시 슬라이드 생성 전에 throw하여 빈 PPTX 생성됨
- body background를 단색(#1A1640)으로 변경, 그래디언트는 convert 스크립트에서 PNG로 덮어쓰기
- 구분선 gradient도 solid rgba로 교체

sam/docs/brochure/v5/slides/brochure-dashboard-1page.html

@@ -7,7 +7,7 @@
     body {
       width: 405pt; height: 720pt;
       font-family: 'Pretendard', sans-serif;
-      background: linear-gradient(175deg, #0F172A 0%, #1E1B4B 55%, #312E81 100%);
+      background: #1A1640;
       padding: 22pt 20pt 12pt 20pt;
       display: flex; flex-direction: column;
     }

sam/docs/brochure/v5/slides/brochure-dashboard-back.html

@@ -7,7 +7,7 @@
     body {
       width: 405pt; height: 720pt;
       font-family: 'Pretendard', sans-serif;
-      background: linear-gradient(175deg, #0F172A 0%, #1E1B4B 55%, #312E81 100%);
+      background: #1A1640;
       padding: 20pt 22pt 12pt 22pt;
       display: flex; flex-direction: column;
     }

sam/docs/brochure/v5/slides/brochure-dashboard-front.html

@@ -7,7 +7,7 @@
     body {
       width: 405pt; height: 720pt;
       font-family: 'Pretendard', sans-serif;
-      background: linear-gradient(175deg, #0F172A 0%, #1E1B4B 55%, #312E81 100%);
+      background: #1A1640;
       padding: 26pt 22pt 16pt 22pt;
       display: flex; flex-direction: column;
     }