* fix: allow deleting walls and slabs via floating action menu
Walls and slabs were excluded from ALLOWED_TYPES in FloatingActionMenu,
so no delete UI appeared when selecting them. Added wall/slab to the
allowed types and show only the delete button (no move/duplicate) since
they are structural elements.
Usage: switch to select mode (V), click a wall or slab, then use the
floating trash icon or press Delete/Backspace to remove it.
Closes#156
* fix: offset delete button higher on walls/slabs to avoid covering measurement labels
* feat: implement sledgehammer-style delete mode
Clicking the trash icon in the floating action menu now activates a
persistent delete mode (like The Sims sledgehammer tool) instead of
deleting immediately. Users can then click objects on the canvas to
delete them one by one. Press V or Escape to exit delete mode.
- Wire delete mode clicks/hover in SelectionManager
- Add D keyboard shortcut to activate delete mode
- Change floating menu delete button to activate delete mode
- Add crosshair cursor when delete mode is active
