Belt printer: declare up_direction in both phong fragment shaders

#16195 added slope.up_direction to the SlopeDetection uniform struct of
phong.vs (110 and 140) but not to phong.fs, so the two stages declared the
uniform with different types and the program failed to link: "unable to
load shaders: phong" at startup, and studio lighting / realistic phong
rendering fell back. gouraud.fs already carries the member.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

resources/shaders/110/phong.fs

@@ -41,6 +41,7 @@ struct SlopeDetection
     bool actived;
     float normal_z;
     mat3 volume_world_normal_matrix;
+    vec3 up_direction;
 };
 
 uniform vec4 uniform_color;

resources/shaders/140/phong.fs

@@ -44,6 +44,7 @@ struct SlopeDetection
     bool actived;
     float normal_z;
     mat3 volume_world_normal_matrix;
+    vec3 up_direction;
 };
 
 uniform vec4 uniform_color;