fix(view): move the ③ colour control into the 3D viewer window

The colour basis control and legend sit below the 3D canvas and show only on ③, so they belong to the model window.

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

src/style.css

@@ -705,3 +705,4 @@ body { background: #eef1f4; font-family: 'Noto Sans KR', system-ui, sans-serif;
 .rebar-legend { display: flex; flex-wrap: wrap; gap: 10px; list-style: none; margin: 0; padding: 0; font-size: 13px; }
 .rebar-legend-item { display: flex; align-items: center; gap: 6px; }
 .rebar-legend .swatch { display: inline-block; width: 14px; height: 14px; border-radius: 3px; }
+.rebar-color[hidden] { display: none; }