feat(styles): make style managers transactional — persist only on Apply

A style manager is now a staged transaction. When it opens, a snapshot of
the style tables, style objects and current-style pointers is taken. Every
New / Copy / Delete / Rename / Set Current / property edit mutates the
document live so the dialog previews the result, but the tab is not marked
dirty, nothing is pushed to undo, and the drawing is not rebuilt.

- Apply commits: one undo entry (restoring the pre-edit state), the tab is
  marked dirty, the drawing rebuilds, and the baseline is reset so further
  edits stage afresh.
- Closing the window without Apply discards: the opening snapshot is
  restored, so none of the changes persist.

Side effects (dirty / undo / bump_geometry) were removed from every style
edit handler and routed exclusively through the commit. Multiline's Apply
is a structural-only commit since its editor has no editable properties.

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

src/ui/tablestyle.rs

@@ -3,7 +3,6 @@
 use crate::app::Message;
 use iced::widget::{
     button, checkbox, column, container, pick_list, row, scrollable, text, text_input, Column,
-    Space,
 };
 use iced::{Background, Border, Color, Element, Fill, Theme};