refactor(entities): collocate ui_name + dxf_name into entities/names.rs

Two near-identical match-on-EntityType tables lived far apart:
scene/properties.rs::entity_type_name returned mixed-case UI labels;
app/commands.rs::entity_type_name returned uppercase DXF record names.
Both enumerated ~30 variants; adding a new entity meant remembering to
touch two files in two crates' worth of source layout.

Move both into a single entities/names.rs as `ui_name()` and
`dxf_name()`. Adjacent so a new variant gets both labels added at once;
function names make the intended audience explicit at the callsite.

scene/properties.rs and app/commands.rs each lose their local match
plus one re-export, gaining a `crate::entities::names::*` call.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/helpers.rs

@@ -164,7 +164,7 @@ pub(super) fn next_group_auto_name(scene: &crate::scene::Scene) -> String {
 // ── Entity type labels ─────────────────────────────────────────────────────
 
 pub(super) fn entity_type_label(entity: &acadrust::EntityType) -> String {
-    crate::scene::properties::entity_type_name(entity).to_string()
+    crate::entities::names::ui_name(entity).to_string()
 }
 
 pub(super) fn entity_type_key(entity: &acadrust::EntityType) -> String {