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/ui/mod.rs

@@ -2,33 +2,24 @@
 /// Change this to scale the ribbon, layer manager rows, and property panel rows uniformly.
 pub const ROW_H: f32 = 26.0;
 
-pub mod about;
 pub mod app_menu;
 pub mod color_select;
 pub mod command_line;
 pub mod icons;
-pub mod layers;
-pub mod layout_manager;
 pub mod modal;
-pub mod open_progress;
 pub mod overlay;
-pub mod page_setup;
-pub mod plugin_manager;
 pub mod popup;
 pub mod properties;
 pub mod ribbon;
 pub mod side_toolbar;
-pub mod shortcuts;
 pub mod statusbar;
-pub mod statusbar_config;
-pub mod statusbar_menu;
 pub mod style;
 pub mod text_util;
-pub mod update_notice;
+pub mod window;
 
 pub use app_menu::AppMenu;
 pub use command_line::CommandLine;
-pub use layers::LayerPanel;
 pub use properties::PropertiesPanel;
 pub use ribbon::Ribbon;
 pub use statusbar::StatusBar;
+pub use window::layers::LayerPanel;