Select Color: reuse helpers instead of repeated code

- book::add_or_replace adds a colour book or replaces one of the same
  name, for both reading the folders and loading a file.
- style.rs: the Plot Style and View Background colour targets use
  math::color_rgb (and hex_string) instead of their own conversions.
- Ribbon: one layer_color lookup for Ribbon::active_layer_color and the
  ribbon's ByLayer swatch.
- Picker: one swatch() for the Index and Document swatches, fill_of() for
  "colour or gray" and divider() for the 1 px separators.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/app/update/mod.rs

@@ -12004,17 +12004,10 @@ impl OpenCADStudio {
             }
             Message::ColorBookLoaded(Ok(book)) => {
                 // A book loaded again replaces the earlier copy.
-                let books = std::sync::Arc::make_mut(&mut self.color_books);
-                let index = match books.iter().position(|b| b.name == book.name) {
-                    Some(index) => {
-                        books[index] = book;
-                        index
-                    }
-                    None => {
-                        books.push(book);
-                        books.len() - 1
-                    }
-                };
+                let index = crate::ui::color_select::book::add_or_replace(
+                    std::sync::Arc::make_mut(&mut self.color_books),
+                    book,
+                );
                 if let Some(picker) = self.color_picker.as_mut() {
                     picker.show_book(std::sync::Arc::clone(&self.color_books), index);
                 }
@@ -12162,10 +12155,7 @@ impl OpenCADStudio {
             .filter_map(|path| std::fs::read_to_string(path).ok())
             .filter_map(|text| book::parse(&text).ok())
         {
-            match books.iter_mut().find(|b| b.name == loaded.name) {
-                Some(same) => *same = loaded,
-                None => books.push(loaded),
-            }
+            book::add_or_replace(&mut books, loaded);
         }
         self.color_books = std::sync::Arc::new(books);
     }

src/app/update/style.rs

@@ -1171,14 +1171,8 @@ impl OpenCADStudio {
         ) {
             self.color_picker = None;
 
-            let rgb = match color {
-                codec::types::Color::Rgb { r, g, b } => Some((r, g, b)),
-                codec::types::Color::Index(index) => codec::types::aci_table::aci_to_rgb(index),
-                _ => None,
-            };
-
-            if let Some((r, g, b)) = rgb {
-                self.ps_color_buf = format!("#{r:02X}{g:02X}{b:02X}");
+            if let Some((r, g, b)) = crate::ui::color_select::math::color_rgb(color) {
+                self.ps_color_buf = crate::ui::color_select::math::hex_string(r, g, b);
             }
 
             return self.on_plot_style_panel_apply();
@@ -1191,15 +1185,11 @@ impl OpenCADStudio {
                 )))
             }
             Some(crate::app::ColorPickTarget::ViewBackground(slot)) => {
-                let rgb = match color {
-                    codec::types::Color::Rgb { r, g, b } => Some([r, g, b]),
-                    codec::types::Color::Index(index) => {
-                        codec::types::aci_table::aci_to_rgb(index).map(|(r, g, b)| [r, g, b])
-                    }
-                    _ => None,
-                };
-                rgb.map(|rgb| {
-                    Message::PdfDialog(crate::ui::window::pdf_dialogs::PdfDialogMsg::BgColor(slot, rgb))
+                crate::ui::color_select::math::color_rgb(color).map(|(r, g, b)| {
+                    Message::PdfDialog(crate::ui::window::pdf_dialogs::PdfDialogMsg::BgColor(
+                        slot,
+                        [r, g, b],
+                    ))
                 })
             }
             Some(crate::app::ColorPickTarget::GraphicAttributesLine) => Some(