fix: make Html occlusion material transparent

[raashish1601]

Why

Closes #2352.

Html's default blending occlusion plane uses a shader that writes vec4(0.0, 0.0, 0.0, 0.0), but the generated shaderMaterial was not marked as transparent. That means the alpha channel can be ignored by the material pipeline, leaving an opaque backing square behind simple HTML content.

What

Mark the default occlusion shaderMaterial as transparent so its zero-alpha fragment is actually blended away. Custom material props are left untouched.

Checks

• corepack yarn prettier --check src/web/Html.tsx
• corepack yarn eslint src/web/Html.tsx
• corepack yarn typecheck
• corepack yarn build (passes with existing Rollup unused-import warnings and outdated Browserslist data warning)
• git diff --check

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