feat(rebar): select several bars on ③ and show only the selected ones (선택만 보기)

The 철근표 has a selection checkbox per row. Select-all / clear and 선택만 보기 sit above the table. In 선택만 보기 the other
bars are hidden in 3D and the members stay at 50% for context. A 3D click or an alert [보기] selects one bar; the
selection clears when the member tab or the pier changes. The selection is screen state and is not saved to the project.

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

src/style.css

@@ -707,3 +707,5 @@ body { background: #eef1f4; font-family: 'Noto Sans KR', system-ui, sans-serif;
 .rebar-legend .swatch { display: inline-block; width: 14px; height: 14px; border-radius: 3px; }
 .rebar-color[hidden] { display: none; }
 .viewer-head .rebar-color { margin: 0 auto; }
+.rebar-select { display: flex; align-items: center; gap: 8px; flex-wrap: wrap; margin: 6px 0; }
+.rebar-select .btn { min-height: 44px; }