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/update.rs

@@ -4971,6 +4971,16 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::InvertSelection => {
+                let i = self.active_tab;
+                self.tabs[i].scene.selection.borrow_mut().context_menu = None;
+                let count = self.tabs[i].scene.invert_selection();
+                self.command_line
+                    .push_output(&format!("Invert Selection: {} object(s) selected.", count));
+                self.refresh_properties();
+                Task::none()
+            }
+
             Message::QSelectOpen => {
                 let i = self.active_tab;
                 self.tabs[i].scene.selection.borrow_mut().context_menu = None;