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/style/form.rs

@@ -83,6 +83,12 @@ pub fn vseparator<'a>(height: Length) -> Element<'a, Message> {
         .into()
 }
 
+/// Alias kept for call sites that historically used `vsep`.
+/// Same implementation as [`vseparator`].
+pub fn vsep<'a>(height: Length) -> Element<'a, Message> {
+    vseparator(height)
+}
+
 /// Muted section heading.
 pub fn section_label<'a>(label: Cow<'static, str>) -> Element<'a, Message> {
     text(label).size(11).style(muted_style).into()