fix(properties): stop block Attributes section rendering twice

A block reference's Attributes group was built in two places — the Insert
entity's own properties() builder and an older panel injection left from
the issue-192 attribute-editing work — so it appeared twice in the panel.
Both produced identical AttrText rows (get_value() just returns &value).
Drop the panel injection; the entity builder is the single source and
also covers multi-select. Attribute edits still route by tag, unchanged.

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

src/app/properties.rs

@@ -256,30 +256,6 @@ impl OpenCADStudio {
                         }
                     }
 
-                    // Block attributes: expose each attribute tag as an
-                    // editable value row so a block reference's attributes can
-                    // be read and changed directly from the panel.
-                    if let acadrust::EntityType::Insert(ins) = entity {
-                        if !ins.attributes.is_empty() {
-                            let props = ins
-                                .attributes
-                                .iter()
-                                .map(|a| crate::scene::model::object::Property {
-                                    label: a.tag.clone(),
-                                    field: "attr",
-                                    value: crate::scene::model::object::PropValue::AttrText {
-                                        tag: a.tag.clone(),
-                                        value: a.get_value().to_string(),
-                                    },
-                                })
-                                .collect();
-                            sections.push(crate::scene::model::object::PropSection {
-                                title: "Attributes".to_string(),
-                                props,
-                            });
-                        }
-                    }
-
                     // ── Doc-dependent property rows ──────────────────────────
                     // Rows whose value lives on another object (a block record,
                     // an underlay definition, a dimension / multileader style)