feat: add three.js starter for HDRIs (Refs #41)

[KuraPiee]

Description

Closes #41: Adds the Three.js code starter for HDRIs in \src/helpers/code/hdri/three.js.

What was added

• Integrated Three.js \RGBELoader\ and \PMREMGenerator\ to load equirectangular HDRI textures from \hdri.file.
• Set \scene.background = envMap\ and \scene.environment = envMap, matching the R3F starter behavior in \src/helpers/code/hdri/r3f.js.
• Added tone mapping configuration (\THREE.ACESFilmicToneMapping) and exposure.
• Added the preview reflective metallic sphere (\SphereBufferGeometry\ + \MeshStandardMaterial({ metalness: 1, roughness: 0 })).
• Formatted with Prettier according to repository standards.

[SaraVieira]

Thank you so much!