feat(view): show the 3D model on the 형상계획 and 철근배근 screens

The 3D viewer (views, canvas, part toggles) moves into a slot at the top of ②
and ③ while those screens are shown, and back to ① otherwise. Moving the
element keeps the WebGL context; the viewer is measured after its screen is
visible.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

src/style.css

@@ -687,3 +687,7 @@ body { background: #eef1f4; font-family: 'Noto Sans KR', system-ui, sans-serif;
 .rebar-table th:nth-child(9),
 .rebar-table th:nth-child(10) { text-align: right; }
 .rebar-table td:nth-child(12) { white-space: normal; min-width: 240px; }
+/* 3D viewer shared by ①, ②, ③: the slot takes the full width above the other columns */
+.viewer-slot { flex: 1 1 100%; min-width: 0; }
+.viewer-slot > .viewer { width: 100%; }
+