feat(select): invert selection from the right-click menu

Add an Invert Selection entry to the selection context menu (below
Select Similar): it replaces the current selection with every other
selectable object in the active layout. Candidates come from the visible
wire set, so objects on off/frozen layers stay excluded.

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

src/app/view.rs

@@ -2907,6 +2907,10 @@ fn viewport_context_menu_overlay(
             ));
             items.push(sep());
             items.push(item("Select Similar".to_string(), Message::SelectSimilar));
+            items.push(item(
+                "Invert Selection".to_string(),
+                Message::InvertSelection,
+            ));
         }
         if isolation_active {
             items.push(item(