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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/app/mod.rs

@@ -1067,10 +1067,9 @@ pub(super) struct OpenCADStudio {
     // ── Floating panel windows ────────────────────────────────────────────
     /// The open Select Color panel, if any.
     color_picker: Option<crate::ui::color_select::ColorPicker>,
-    /// The built-in colour books and those in `color_book_dirs`, read
-    /// lazily when the Select Color panel opens.
+    /// The built-in colour book and those in the colour book folder, read
+    /// lazily when the Select Color panel first opens.
     color_books: std::sync::Arc<Vec<crate::ui::color_select::book::ColorBook>>,
-    color_book_dirs: Vec<String>,
     color_books_read: bool,
     /// Recently committed real colours, newest first.
     /// ACI and True Color remain semantically distinct even when their RGB matches.
@@ -4793,7 +4792,6 @@ impl OpenCADStudio {
             web_autosave_clear_after: Vec::new(),
             color_picker: None,
             color_books: Default::default(),
-            color_book_dirs: Vec::new(),
             color_books_read: false,
             active_modal: None,
             hyperlink_editor_handles: Vec::new(),