Skins: Thumbnails show white line at bottom

[sampotts]

In certain browsers (e.g. Chrome), we end up with fractional pixels for the dimensions of the thumbnail container, resulting in light lines at the bottom of the thumbnail.

[github-actions]

Planning recommendation

This is a follow-on visual bug from the thumbnail skin work completed in #814. The fix likely lives in the thumbnail CSS within the skin files (packages/html and packages/react), applying overflow: hidden or rounding dimensions to avoid subpixel gaps (similar to the recent fix in #993 for the minimal video skin).

• Roadmap: Worth adding to the skin polish wave alongside other skin refinements.
• Related: fix(skin): remove overflow in minimal video skin #993 (overflow fix in minimal skin) and Skins: Add slider time preview and thumbs #814 (thumbnail skin feature) are the closest prior art.
• Size: Should be a small CSS-only fix (size-XS or size-S).