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/style/mod.rs

@@ -10,6 +10,7 @@ pub mod scale_manager;
 pub mod anno_object_scale;
 pub mod style_manager;
 pub mod common;
+pub mod row_button;
 pub mod form;
 pub mod fusion_theme;
 

src/ui/style/row_button.rs

@@ -0,0 +1,57 @@
+//! Row/header button styles shared by the layers and xref list panels.
+//!
+//! Centralizes the zebra/selected row-button coloring previously duplicated
+//! in `src/ui/window/layers.rs` (`layer_cell_button_style` +
+//! `layer_header_button_style`) and `src/ui/window/xref_manager.rs`
+//! (`row_button_style`). The two cell copies were functionally byte-identical
+//! (Hovered → `background.strong`, selected → `primary.weak`, even →
+//! `background.base`, else → `background.weak`); a single source keeps hover,
+//! selection, and zebra colors consistent across both panels.
+
+use iced::widget::button;
+use iced::{Background, Theme};
+
+/// Cell (row) button style — closure form so both `.style(...)` call sites
+/// (`layers.rs`, `xref_manager.rs`) keep their existing shape.
+pub(crate) fn cell_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()
+        }
+    }
+}
+
+/// Column-header button style (layers panel).
+pub(crate) fn header_button_style(theme: &Theme, status: button::Status) -> button::Style {
+    let palette = theme.palette();
+    let highlighted = matches!(
+        status,
+        button::Status::Hovered | button::Status::Pressed
+    );
+    let pair = if highlighted {
+        palette.background.strong
+    } else {
+        palette.background.weak
+    };
+    button::Style {
+        background: highlighted.then_some(Background::Color(pair.color)),
+        text_color: pair.text,
+        ..Default::default()
+    }
+}