refactor: group ui/ style+popup dirs, rename cmd_result driver

- Group the flat src/ui/ dir: move the 9 style editors into ui/style/
  (dimstyle, mleaderstyle, mlstyle, plotstyle, textstyle, tablestyle,
  point_style, style_list, style_manager) and the 6 popups into
  ui/popup/ (cycle, isolate, scale, selection_filter, snap, units).
  Call sites repointed to crate::ui::style::/crate::ui::popup::; no
  compat re-exports. ui/ root drops from 36 to 21 files.
- Rename app/cmd_result.rs -> app/command_driver.rs: the file is the
  StepInput->feed_command driver, not the CmdResult type (which lives
  in command.rs). Name now matches contents.

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

src/app/mod.rs

@@ -2,7 +2,7 @@
 mod automation;
 #[cfg(not(target_arch = "wasm32"))]
 pub use automation::{export_headless, serve};
-mod cmd_result;
+mod command_driver;
 mod commands;
 mod document;
 mod expr_eval;