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/shape.rs

@@ -144,7 +144,6 @@ impl PropertyEditable for Shape {
                 edit("Rotation", "shp_rot", self.rotation.to_degrees()),
                 edit("Width Factor", "shp_xs", self.relative_x_scale),
                 edit("Oblique Angle", "shp_ob", self.oblique_angle.to_degrees()),
-                edit("Thickness", "shp_th", self.thickness),
                 edit("Normal X", "shp_nx", self.normal.x),
                 edit("Normal Y", "shp_ny", self.normal.y),
                 edit("Normal Z", "shp_nz", self.normal.z),
@@ -168,7 +167,6 @@ impl PropertyEditable for Shape {
             "shp_rot" => self.rotation = v.to_radians(),
             "shp_xs" if v.abs() > 1e-9 => self.relative_x_scale = v,
             "shp_ob" => self.oblique_angle = v.to_radians(),
-            "shp_th" => self.thickness = v,
             "shp_nx" => self.normal.x = v,
             "shp_ny" => self.normal.y = v,
             "shp_nz" => self.normal.z = v,