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/locales/ja.json

@@ -110,7 +110,6 @@
   "ui.front": "正面",
   "ui.fully_constrained": "完全拘束",
   "ui.general": "一般",
-  "ui.generative_design": "ジェネレーティブ デザイン",
   "ui.graphics": "グラフィックス",
   "ui.heading_assemble": "アセンブリ",
   "ui.heading_box": "直方体",