feat: a Show Grid switch in the viewport menu's Guides group

The existing toggle_grid command as a marked row ahead of Show Origin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

 CLAUDE.md   |  3 ++-
 src/app.rs  |  5 +++--
 src/main.rs | 23 ++++++++++++++++++++++-
 3 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/CLAUDE.md b/CLAUDE.md
index d242972..f48445e 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1377,7 +1377,8 @@ multi-selection, so taking `Shift+L` now would have to be given back later.
 ### Display mode: the viewport menu, and smooth shading
 
 The viewport's right-click menu carries the DISPLAY MODE under Frame All
-and View 1:1, in five groups a separator apart — guides (Show Origin, the
+and View 1:1, in five groups a separator apart — guides (Show Grid, Show
+Origin, the
 scene furniture that is not the geometry); wireframe (switch,
 thickness); points (Show Points, Point Size, and Group Marker Scale, which
 multiplies it); overlays (Show Point Markers and its size, Show Point
diff --git a/src/app.rs b/src/app.rs
index b187b96..1e0ad84 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -4462,8 +4462,8 @@ impl State {
     }
 
     /// The viewport menu's rows and what each does, in groups a separator
-    /// apart: framing; the GUIDES (Show Origin — the scene furniture that
-    /// is not the geometry); the WIREFRAME (its switch and thickness); the
+    /// apart: framing; the GUIDES (Show Grid, Show Origin — the scene
+    /// furniture that is not the geometry); the WIREFRAME (its switch and thickness); the
     /// POINTS
     /// (the Show Points switch, point size, and the group marker scale that
     /// multiplies it); the OVERLAYS (Show Point Markers and its size, Show
@@ -4486,6 +4486,7 @@ impl State {
 
         // Guides: the scene furniture that is not the geometry.
         row(&mut options, &mut actions, "-".into(), sep);
+        row(&mut options, &mut actions, format!("{} {}", mark(self.viewport().show_grid), label("toggle_grid", "Show Grid")), ViewportMenuAction::Command("toggle_grid"));
         row(&mut options, &mut actions, format!("{} {}", mark(self.viewport().show_origin), label("toggle_origin", "Show Origin")), ViewportMenuAction::Command("toggle_origin"));
 
         // Wireframe.
diff --git a/src/main.rs b/src/main.rs
index 062419a..a20d304 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1658,7 +1658,7 @@ mod tests {
             .map(|g| g.to_vec())
             .collect();
         assert_eq!(groups[0], vec![A::FrameAll, A::OneToOne]);
-        assert_eq!(groups[1], vec![A::Command("toggle_origin")]);
+        assert_eq!(groups[1], vec![A::Command("toggle_grid"), A::Command("toggle_origin")]);
         assert_eq!(groups[2], vec![A::Command("toggle_wireframe"), A::WireThicknessSlider]);
         assert_eq!(
             groups[3],
@@ -1681,6 +1681,27 @@ mod tests {
         assert!(options.iter().zip(&actions).all(|(o, a)| (o == "-") == (*a == A::Separator)), "separator rows line up");
     }
 
+    /// Show Grid heads the Guides group, marked from the live flag; the row
+    /// runs the command.
+    #[test]
+    fn the_viewport_menu_toggles_show_grid() {
+        use crate::app::ViewportMenuAction as A;
+        let mut state = State::new(false);
+        state.viewport_mut().show_grid = true;
+        let row = |state: &State| {
+            let (options, actions) = state.viewport_menu_rows();
+            let i = actions.iter().position(|a| *a == A::Command("toggle_grid")).expect("a Show Grid row");
+            options[i].clone()
+        };
+        let label = crate::command::by_id("toggle_grid").unwrap().label;
+        assert_eq!(row(&state), format!("● {label}"));
+        state.run_viewport_menu_action(A::Command("toggle_grid"));
+        assert!(!state.viewport().show_grid);
+        assert_eq!(row(&state), format!("○ {label}"));
+        let kdl = fs::read_to_string(crate::app::DesignSettings::file_path()).expect("saved");
+        assert!(!crate::app::DesignSettings::from_kdl_str(&kdl).viewport.show_grid_enabled, "persisted");
+    }
+
     /// Show Origin is a switch in the Guides group, marked from the live
     /// flag; the row runs the command, which also keeps the viewport
     /// menubar's Guides checkmark in step.