Select Color: colour book folders, app-style view toggles, load button by the book list

- Colour books now come from folders, like AutoCAD's Color Book
  Locations: Options > Files lists them (add, open, remove); by default
  one color_books folder in the config directory. The Color Books tab
  shows the built-in books plus every .acb in those folders; Load copies
  the chosen file into the first folder. Web: session only, as before.
- Grid/list buttons use the app's toggle style (light accent, its text
  colour for the icon, same corners as inactive).
- Color Books: the load button sits right of the book list; the search
  box keeps its start over the page strip and runs to the end.
- Translations for the two new Options strings in all 22 locales.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/app/mod.rs

@@ -965,9 +965,10 @@ pub(super) struct OpenCADStudio {
     // ── Floating panel windows ────────────────────────────────────────────
     /// The open Select Color panel, if any.
     color_picker: Option<crate::ui::color_select::ColorPicker>,
-    /// Colour books loaded this session, read lazily from `color_book_paths`.
+    /// The built-in colour books and those in `color_book_dirs`, read
+    /// lazily when the Select Color panel opens.
     color_books: std::sync::Arc<Vec<crate::ui::color_select::book::ColorBook>>,
-    color_book_paths: Vec<String>,
+    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.
@@ -4174,8 +4175,13 @@ pub enum Message {
     /// Commit the colour chosen in the shared picker, with its `BOOK$NAME`
     /// when it comes from a colour book.
     ColorWindowPick(codec::types::Color, Option<String>),
-    /// An `.acb` file was read: the book and, on desktop, where it lives.
-    ColorBookLoaded(Result<(crate::ui::color_select::book::ColorBook, Option<String>), String>),
+    /// An `.acb` file was read (and, on desktop, copied into the first
+    /// colour book folder).
+    ColorBookLoaded(Result<crate::ui::color_select::book::ColorBook, String>),
+    /// Options › Files: pick a folder to search for colour books.
+    ColorBookDirBrowse,
+    ColorBookDirAdd(String),
+    ColorBookDirRemove(usize),
     /// Set a block/linetype Handle field on the selected dim style from a
     /// dropdown of available block-records / linetypes (by name).
     DsSetHandle {
@@ -4608,7 +4614,7 @@ impl OpenCADStudio {
             web_autosave_clear_after: Vec::new(),
             color_picker: None,
             color_books: Default::default(),
-            color_book_paths: Vec::new(),
+            color_book_dirs: Vec::new(),
             color_books_read: false,
             active_modal: None,
             hyperlink_editor_handles: Vec::new(),