Remove the Generative Design workspace placeholder (#412)

It isn't planned, so it no longer appears in the workspace menu's
"coming later" list. Drops ui.generative_design from every locale and
checks the menu doesn't list it.


Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/menu_tests.rs

@@ -616,4 +616,5 @@ fn the_workspace_menu_lists_design_and_greys_the_rest() {
     let items = context_menu::items(&app, &Target::Workspace);
     assert!(find(&items, "Design").enabled);
     assert!(!find(&items, "Render").enabled && find(&items, "Render").shortcut == "coming later");
+    assert!(!items.iter().any(|i| i.label.contains("Generative")));
 }