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/update/mod.rs

@@ -954,7 +954,7 @@ impl OpenCADStudio {
                         if let Some(dl) = self.tabs[i].scene.document.layers.get_mut(&name) {
                             dl.color = AcadColor::Index(aci);
                         }
-                        use crate::ui::layers::iced_color_from_acad;
+                        use crate::ui::window::layers::iced_color_from_acad;
                         let new_color = iced_color_from_acad(&AcadColor::Index(aci));
                         if let Some(pl) = self.tabs[i].layers.layers.get_mut(idx) {
                             pl.color = new_color;