Home: turn off commands that act on the hidden design (#322)

The Home page hides the open design, yet Save, Undo, Redo, File › Insert,
Save As and Export, and the design's keyboard shortcuts (Ctrl+S/Z/Y,
Delete, V, F2, F6, Esc, S and tool keys such as E) still acted on it:
Delete removed the selected body unseen, Save wrote a design the user
could not see, and tool keys left Home without being asked to.

On Home the top bar's Save, Undo and Redo are dimmed with a translated
"open a design first" tooltip, the File menu's design items are greyed
out, and only New, Open and Preferences keep their shortcuts. Everything
works as before once Home is closed.

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

crates/ui-egui/src/locales/de.json

@@ -187,6 +187,7 @@
   "ui.multiple": "Mehrfach",
   "ui.named_views": "Benannte Ansichten",
   "ui.navigation": "Navigation",
+  "ui.needs_design": "{command}: Öffnen Sie zuerst ein Design",
   "ui.new_body": "Neuer Körper",
   "ui.new_design": "Neues Design",
   "ui.normal": "Normal",