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/layout_manager.rs

@@ -1,6 +1,7 @@
 //! Layout Manager window — fills the entire OS window.
 
 use crate::app::Message;
+use crate::ui::style::form::{hdivider, vsep};
 use iced::widget::{button, column, container, row, scrollable, text, text_input, Space};
 use iced::{Background, Element, Theme};
 use crate::t;
@@ -37,32 +38,6 @@ fn list_item(active: bool) -> impl Fn(&Theme, button::Status) -> button::Style {
     }
 }
 
-fn hdivider<'a>(width: iced::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()
-}
-
-fn vsep<'a>(height: iced::Length) -> Element<'a, Message> {
-    container(Space::new().width(1).height(height))
-        .width(1)
-        .height(height)
-        .style(|theme: &Theme| container::Style {
-            background: Some(Background::Color(
-                theme.palette().background.neutral.color,
-            )),
-            ..Default::default()
-        })
-        .into()
-}
-
 pub fn view_window<'a>(
     layouts: Vec<String>,
     selected: &'a str,