Select Color: one fixed colour book folder instead of a folder list setting

The color_book_dirs setting is gone. Colour books live in color_books in
the config folder (book::folder()), like the other fixed locations;
Options > Files shows it through Folders::color_books. read_all() and
load_from_dialog() use that folder directly.

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

src/app/view/modal.rs

@@ -373,6 +373,8 @@ impl OpenCADStudio {
                                         .map(|_| std::env::temp_dir().display().to_string()),
                                     fonts: crate::io::font_repo::fonts_dir()
                                         .map(|p| p.display().to_string()),
+                                    color_books: crate::ui::color_select::book::folder()
+                                        .map(|p| p.display().to_string()),
                                 }
                             }
                             #[cfg(target_arch = "wasm32")]
@@ -381,7 +383,6 @@ impl OpenCADStudio {
                             }
                         },
                         &self.qnew_template,
-                        &self.color_book_dirs,
                         self.double_click_block_refedit,
                         self.double_click_block_attedit,
                         self.cursor_type,