Draw gradient backgrounds as the reference does

The rotation is stored in degrees and turns the bottom-to-top direction
counter-clockwise, measured in pixels; a two-colour gradient blends from
the bottom colour to the top one across the whole viewport.

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

src/scene/view/render.rs

@@ -2982,7 +2982,8 @@ impl Scene {
                     2.0,
                     background.horizon as f32,
                     background.height as f32,
-                    background.rotation as f32,
+                    // Stored in degrees.
+                    (background.rotation as f32).to_radians(),
                 ];
                 result
             }