Two fixes for stair selection in 2D floorplan not working:
1. floorplan-panel: remove level > 0 restriction on auto-switch to structure
phase. When viewing ANY level in the floorplan while in site phase, auto-enter
structure mode so structural nodes (stairs, walls, slabs) become selectable.
2. selection-manager: expand auto-switch to also cover site phase. Clicking a
structural or furnish element now auto-switches from site → structure/furnish,
just like it already did between structure ↔ furnish ↔ zones.
This fixes the issue where stairs couldn't be selected in 2D and the floating
action menu (move/duplicate/delete) wouldn't appear.
Pascal
6c3ac3e030