refactor: deduplicate entity type name mapping

Make scene::properties::entity_type_name the single source of truth.
entity_type_label in helpers.rs now delegates to it instead of
maintaining a separate match list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scene/properties.rs

@@ -67,7 +67,7 @@ pub fn fallback_properties(_handle: Handle, entity: &EntityType) -> PropSection
     }
 }
 
-fn entity_type_name(e: &EntityType) -> &'static str {
+pub fn entity_type_name(e: &EntityType) -> &'static str {
     match e {
         EntityType::Point(_) => "Point",
         EntityType::Line(_) => "Line",