Add environment map and metallic reflections to html-texture example

[mvaligursky]

Add environment map and metallic material to the html-texture example for visually richer output.

Changes:

• Load the helipad environment atlas and set up a skybox with reflections
• Switch cube material to metalness workflow (metalness 0.7, gloss 0.9) for mirror-like environment reflections tinted by the HTML texture
• Wrap scene setup in AssetListLoader callback for proper asset loading
• Update camera clear color and directional light for better visual contrast with the skybox

Examples:

• Updated misc/html-texture example