Viewer: a periodic calculation opens without its cell drawn (Periodic Builder off) #355

Found 2026-10-15 (Linux, test-912 build, QE MD on 8-atom silicon). Opening the calculation in the Viewer, while running or finished, shows the atoms without the unit cell. The Periodic Builder pane is off. Turning it on shows the cell, and the cell then stays on every Geometry Trace frame, so the data is there.

Expected: a calculation whose structure has a cell (QE, and any periodic import) opens with the cell drawn, as the Builder does for a QE calculation. Since 33c4019 the Periodic Builder pane is no longer restored from the saved layout, so this has to follow from the structure. Not release-critical.