feat: Add translucent view cube with hover opacity effect

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/shaders/texture.frag

@@ -5,8 +5,10 @@ out vec4 FragColor;
 in vec2 TexCoord;
 
 uniform sampler2D texture_diffuse1;
+uniform float opacity;
 
 void main()
 {
-    FragColor = texture(texture_diffuse1, TexCoord);
+    vec4 texColor = texture(texture_diffuse1, TexCoord);
+    FragColor = vec4(texColor.rgb, texColor.a * opacity);
 }