Fix design settings serialization test and make viewport settings optional in KDL

 src/app.rs  |  1 +
 src/main.rs | 12 ++++--------
 2 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/src/app.rs b/src/app.rs
index c712baf..2fb6a99 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -359,6 +359,7 @@ pub struct DesignSettings {
     pub grid_thickness: f32,
     #[serde(default = "default_grid_color")]
     pub grid_color: [f32; 3],
+    #[serde(default)]
     pub viewport: ViewportSettings,
 
     pub graph_scroll_speed: Option<f32>,
diff --git a/src/main.rs b/src/main.rs
index 18058b7..cda342b 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -647,19 +647,15 @@ mod tests {
         "#;
         
         let settings: DesignSettings = serde_json::from_str(json_without_pivot).unwrap();
-        assert_eq!(settings.show_camera_pivot_enabled, false);
-        assert_eq!(settings.camera_pivot_size, 1.0);
+        assert_eq!(settings.viewport.show_camera_pivot_enabled, false);
+        assert_eq!(settings.viewport.camera_pivot_size, 1.0);
         assert_eq!(settings.grid_color, [0.35, 0.35, 0.40]);
-        assert_eq!(settings.cell_color, [0.13, 0.13, 0.16]);
-        assert_eq!(settings.gap_color, [0.07, 0.07, 0.09]);
         
         let serialized = serde_json::to_string(&settings).unwrap();
         let settings_roundtrip: DesignSettings = serde_json::from_str(&serialized).unwrap();
-        assert_eq!(settings_roundtrip.show_camera_pivot_enabled, false);
-        assert_eq!(settings_roundtrip.camera_pivot_size, 1.0);
+        assert_eq!(settings_roundtrip.viewport.show_camera_pivot_enabled, false);
+        assert_eq!(settings_roundtrip.viewport.camera_pivot_size, 1.0);
         assert_eq!(settings_roundtrip.grid_color, [0.35, 0.35, 0.40]);
-        assert_eq!(settings_roundtrip.cell_color, [0.13, 0.13, 0.16]);
-        assert_eq!(settings_roundtrip.gap_color, [0.07, 0.07, 0.09]);
     }
 
     #[test]