feat: add Phong specular lighting to fragment shader with shininess and strength controls

shaders/simple.frag

@@ -14,5 +14,13 @@ void main() {
     vec3 diffuse = vColor.rgb * lambert;
     // combine ambient and diffuse
     vec3 result = ambient + diffuse;
+    // specular component (Phong)
+    float shininess = 32.0;
+    vec3 viewDir     = vec3(0.0, 0.0, 1.0);
+    vec3 reflectDir  = reflect(-lightDirection, normalize(vNormal));
+    float spec       = pow(max(dot(reflectDir, viewDir), 0.0), shininess);
+    float specStrength = 0.5;
+    vec3 specular    = specStrength * spec * vec3(1.0);
+    result += specular;
     color = vec4(result, vColor.a);
 }