Read Mode: a Page color menu shows the columns on white, sepia or a dark page

Word's Read Mode has View › Page Color with None, Sepia and Inverse.
The Read Mode toolbar now has a Page color menu beside Line spacing,
backed by the view.readPageColor command. None draws the columns on
white and does not show the document's page color or page picture.
Sepia draws them on Word's sepia paper (RGB 112, 66, 20 at 80%
transparency over white) and leaves the text's colors alone. Inverse
draws the dark page that View › Switch Modes draws.

The setting is screen only, is not saved, and starts at None on each
entry to Read Mode, as the other reading controls do. In Read Mode it
sets the paper even while Switch Modes is on. Print Layout and the
other views still show the document's page color, and Switch Modes
still darkens them.

crates/engine/src/session.rs

@@ -178,6 +178,9 @@ pub struct ViewState {
     pub read_zoom: f32,
     #[serde(skip)]
     pub read_line_spacing: Option<f32>,
+    /// Read Mode's Page Color: the paper the reading columns are drawn on.
+    #[serde(skip)]
+    pub read_page_color: crate::cmd::view::ReadPageColor,
     pub dark_mode: bool,
     pub show_markup: bool,
     pub track_changes_pane: bool,
@@ -265,6 +268,7 @@ impl Default for ViewState {
             read_column_width: None,
             read_zoom: default_read_zoom(),
             read_line_spacing: None,
+            read_page_color: Default::default(),
             dark_mode: false,
             show_markup: true,
             track_changes_pane: false,