refactor: group ui/ statusbar and window subdirs

Continue tidying the flat ui/ root:

- ui/statusbar/: statusbar.rs becomes statusbar/mod.rs (its
  crate::ui::statusbar::* paths are preserved) and absorbs
  statusbar_config.rs + statusbar_menu.rs.
- ui/window/: the standalone manager/dialog windows move here —
  about, layout_manager, page_setup, plugin_manager, shortcuts,
  layers, update_notice, open_progress. The crate::ui::LayerPanel
  re-export is repointed so callers are unchanged.

Call sites repointed to the new paths; no compat re-exports beyond the
existing LayerPanel one. ui/ root drops from 21 to 10 flat files with
five subdirs (ribbon, style, popup, statusbar, window).

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

src/app/view/mod.rs

@@ -1083,7 +1083,7 @@ impl OpenCADStudio {
         };
 
         let statusbar_menu_layer: Element<'_, Message> = if self.statusbar_menu_open {
-            crate::ui::statusbar_menu::statusbar_menu_overlay(&self.statusbar_config)
+            crate::ui::statusbar::statusbar_menu::statusbar_menu_overlay(&self.statusbar_config)
         } else {
             iced::widget::Space::new().width(0).height(0).into()
         };
@@ -1141,7 +1141,7 @@ impl OpenCADStudio {
         };
 
         let open_progress_layer: Element<'_, Message> = if let Some(p) = &self.opening {
-            crate::ui::open_progress::view(p, iced::time::Instant::now())
+            crate::ui::window::open_progress::view(p, iced::time::Instant::now())
         } else {
             iced::widget::Space::new().width(0).height(0).into()
         };