feat(properties): show Thickness in the General group

Thickness (DXF 39) is a General-group property in the palette, but it is
stored per-entity, so it was previously shown (inconsistently) only in the
Geometry section of a few entities. Surface it in General for every entity
type that carries an extrusion thickness (arc, circle, line, lwpolyline,
point, polyface mesh, 2D polyline, shape, 2D solid, text) via
entity_thickness / set_entity_thickness, routed through apply_common_prop.

Removes the now-duplicate Thickness rows (and dead apply arms) from the
arc/circle/shape/solid Geometry sections so it is never shown twice.

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

src/entities/solid.rs

@@ -117,7 +117,6 @@ impl PropertyEditable for Solid {
                 edit("Point 4 Y", "sl_p4y", self.fourth_corner.y),
                 edit("Point 4 Z", "sl_p4z", self.fourth_corner.z),
                 ro("Elevation", "sl_elev", format!("{:.4}", elevation)),
-                edit("Thickness", "sl_thick", self.thickness),
             ],
         }]
     }
@@ -139,7 +138,6 @@ impl PropertyEditable for Solid {
             "sl_p4x" => self.fourth_corner.x = v,
             "sl_p4y" => self.fourth_corner.y = v,
             "sl_p4z" => self.fourth_corner.z = v,
-            "sl_thick" => self.thickness = v,
             _ => {}
         }
     }