fix(view): put the ③ colour control in the 3D title row, centred

The colour basis control sits in the 교각 3D 뷰 title row between the 시점 buttons and the origin note.

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

src/style.css

@@ -706,3 +706,4 @@ body { background: #eef1f4; font-family: 'Noto Sans KR', system-ui, sans-serif;
 .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; }
+.viewer-head .rebar-color { margin: 0 auto; }