refactor(ui): centralize hdivider/vsep in style::form

Deduplicate the byte-identical 1px divider helpers previously copy-pasted across 6 modules. form::{hdivider, vseparator} is now canonical, with a form::vsep alias for legacy call sites. style_manager re-exports both so scale_manager and anno_object_scale imports keep working unchanged. Delete locals in style_manager, plotstyle, dimstyle, layout_manager, attribute_editor. Replace thin wrappers in window/plot with direct imports. no visual change.

src/ui/window/attribute_editor.rs

@@ -25,6 +25,7 @@ use std::borrow::Cow;
 use std::fmt;
 
 use crate::ui::properties::{lw_options, LwItem};
+use crate::ui::style::form::hdivider;
 
 const LABEL_W: f32 = 120.0;
 
@@ -161,20 +162,6 @@ fn field_style(theme: &Theme, status: text_input::Status) -> text_input::Style {
     }
 }
 
-/// Horizontal 1px divider in the shared border colour (matches style windows).
-fn hdivider<'a>(width: Length) -> Element<'a, Message> {
-    container(Space::new().width(width).height(1))
-        .width(width)
-        .height(1)
-        .style(|theme: &Theme| container::Style {
-            background: Some(Background::Color(
-                theme.palette().background.neutral.color
-            )),
-            ..Default::default()
-        })
-        .into()
-}
-
 /// One `label : widget` row with a fixed-width label column.
 fn field_row<'a>(
     label: Cow<'static, str>,