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;