feat: add sphere scene with fixed cube-to-sphere mapping

Added scenes/sphere.{hpp,cpp} using the cube-to-sphere
approach from nrz.cpp, but with corrected math: vertices
are simply normalized to project onto the unit sphere
(the original used a broken formula with p=50.0 as an
exponent).

The sphere uses indexed geometry with position, normal,
and UV attributes, plus a simple diffuse lighting shader.

Press 1/2 to switch between cube and sphere scenes.
Updated .gitignore to exclude generated PNG screenshots.

.gitignore

@@ -43,3 +43,6 @@ imgui.ini
 
 # Compiled shaders
 *.spv
+
+# Screenshots
+*.png