fix(lit): replace unsafeCSS with CSSStyleSheet for structural styles

[iamrajhans]

This PR removes unsafeCSS usage from the Lit renderer to fix internal build policy failures reported in issue #462.

Specifically, in renderers/lit/src/0.8/ui/styles.ts, unsafeCSS(Styles.structuralStyles) was replaced with a safer stylesheet construction flow:

• build a CSSStyleSheet
• load Styles.structuralStyles using replaceSync
• return [] as a fallback when CSSStyleSheet is unavailable or fails

This preserves behavior (shared structural styles still applied) while eliminating direct unsafeCSS usage.

Fixes: #462

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

[gemini-code-assist]

Code Review

This pull request effectively addresses the security concern of using unsafeCSS by replacing it with the safer CSSStyleSheet API. The implementation is solid, with a good fallback for environments where CSSStyleSheet is not supported. I have one suggestion to improve debuggability by logging potential errors during stylesheet creation.

[ditman]

I left some comments, and I think @ava-cassiopeia should have final say, but this looks good to me! Thanks for the fix!

(I'm also modifying the description of this PR to link it to the issue about removing unsafeCSS)

[ava-cassiopeia]

This generally LGTM, but I think @ditman has some solid points, so I'll wait to approve until those are addressed.

[iamrajhans]

hi @ava-cassiopeia @ditman could you please review, made the required changes

[ava-cassiopeia]

Thanks for iterating on this, and thanks for your contributions!