fix(thickness): edit persists + wide polyline extrudes as a solid tube

Entering a thickness (DXF 39 extrusion) on a wide polyline now works end
to end:

- The properties Thickness field is a General-group common prop, so it is
  routed to apply_common_prop (set_entity_thickness) instead of the
  geometry setter, which ignored it — the value was silently dropped and
  reverted to 0.
- A wide polyline extrudes its whole band into a 3-D tube (outer + inner
  walls with top and bottom caps), not just its centreline.
- The tube is real 3-D geometry, not a flat draw-order hatch: it renders
  solid in every view mode with true depth, so nothing shows through the
  walls. The flat 2-D band fill is skipped for a thickened polyline.
- A fill whose triangles span depth keeps its real depth (no draw-order
  bias), so the extrusion's own edge wires stay in front of the surface
  and remain visible instead of only the silhouette.
- The corner and cap edge lines read black, framing the coloured tube.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/command.rs

@@ -1576,7 +1576,13 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                                             );
                                         }
                                     }
-                                    "linetype_scale" | "transparency" => {
+                                    "linetype_scale" | "transparency" | "thickness" => {
+                                        // Thickness (DXF 39 extrusion) is a General-
+                                        // group common prop, not a geometry-group one:
+                                        // route it to apply_common_prop (which handles
+                                        // it via set_entity_thickness). apply_geom_prop
+                                        // ignores it, so the edit would otherwise be a
+                                        // silent no-op and revert to the old value.
                                         if let Some(entity) =
                                             self.tabs[i].scene.document.get_entity_mut(handle)
                                         {