fix(viewport): adapt entity colours to visible bg

`model_wires_for_viewport` was tessellating with `self.bg_color` (the
model bg), so model content rendered inside a paper-layout viewport
adapted to a dark bg even though the transparent shader now reveals
the PaperCanvas sheet behind it — white wires stayed white on a light
sheet. Pass the paper bg in a paper layout.

Wire cache also needed an explicit invalidation when the bg or the
active layout changes; otherwise the cached, already-adapted wires
were returned against the new bg unchanged.

- BACKGROUND command bumps `geometry_epoch` after both branches
- new `Scene::set_current_layout` setter bumps on actual change; layout
  switch + history restore use it

src/app/update.rs

@@ -3501,7 +3501,7 @@ impl OpenCADStudio {
                 self.layout_rename_state = None;
                 self.layout_context_menu = None;
                 self.tabs[i].scene.active_viewport = None;
-                self.tabs[i].scene.current_layout = name;
+                self.tabs[i].scene.set_current_layout(name);
                 self.tabs[i].scene.deselect_all();
                 self.tabs[i].scene.restore_saved_camera();
                 self.tabs[i].last_synced_camera_gen = self.tabs[i].scene.camera_generation;