refactor(style): dedup layers/xref row-button style into ui::style::row_button

Moves the byte-identical zebra/selected cell-button bodies from layers.rs and xref_manager.rs plus the layers header-button body into one shared module. All six call sites now use the shared helpers; toolbar_btn helpers untouched (different semantics). No new tests per owner request (pure move, verified by build plus full suite before test removal).

src/ui/window/xref_manager.rs

@@ -10,6 +10,7 @@ use crate::io::xref::collect_entries_with_prev;
 use crate::io::xref_model::{normalize_lexical, Pathtype, RefKind, RefStatus, RefType, ReferenceEntry};
 use crate::ui::ROW_H;
 use crate::ui::style::common::muted_style;
+use crate::ui::style::row_button::cell_button_style;
 use codec::CadDocument;
 use iced::widget::{button, column, container, mouse_area, row, scrollable, text, tooltip};
 use iced::Padding;
@@ -1259,27 +1260,6 @@ fn format_date(modified: Option<SystemTime>) -> String {
 
 // ── Widget helpers (layers.rs conventions) ────────────────────────────────
 
-fn row_button_style(selected: bool, index: usize) -> impl Fn(&Theme, button::Status) -> button::Style {
-    move |theme: &Theme, status: button::Status| {
-        let palette = theme.palette();
-        let highlighted = matches!(status, button::Status::Hovered);
-        let pair = if highlighted {
-            palette.background.strong
-        } else if selected {
-            palette.primary.weak
-        } else if index % 2 == 0 {
-            palette.background.base
-        } else {
-            palette.background.weak
-        };
-        button::Style {
-            background: highlighted.then_some(Background::Color(pair.color)),
-            text_color: pair.text,
-            ..Default::default()
-        }
-    }
-}
-
 fn toolbar_btn(label: String, msg: Option<Message>, fill: bool) -> Element<'static, Message> {
     let mut b = button(text(label).size(11))
         .style(|theme: &Theme, status| {
@@ -1808,7 +1788,7 @@ fn xref_row<'a>(
         let key = entry.key;
         let arrow = button(crate::ui::icons::themed_arrow_toggle(is_expanded, 10.0))
             .on_press(Message::XrefManagerToggleExpand(key))
-            .style(row_button_style(is_selected, display.index))
+            .style(cell_button_style(is_selected, display.index))
             .padding(Padding {
                 top: 6.0,
                 bottom: 6.0,
@@ -1893,7 +1873,7 @@ fn tree_row(
         let key = entry.key;
         let arrow = button(crate::ui::icons::themed_arrow_toggle(is_expanded, 10.0))
             .on_press(Message::XrefManagerToggleExpand(key))
-            .style(row_button_style(is_selected, display.index))
+            .style(cell_button_style(is_selected, display.index))
             .padding(Padding {
                 top: 6.0,
                 bottom: 6.0,