Add soft card variant

[calebporzio]

Summary

• add a soft Card variant for a lower-emphasis surface
• preserve outline as the default Card treatment
• use translucent neutral overlays so the surface adapts to light, dark, and tinted backdrops

Why

An opaque zinc-50/100 treatment looks right on a white page, but can disappear when it is placed on the same zinc surface and can become visually inverted on darker zinc backgrounds.

The soft variant uses translucent black overlays in light mode and white overlays in dark mode. On the base background, these are tuned to visually match the original opaque treatment; on other backgrounds, they retain the backdrop hue while maintaining a consistent surface relationship.

Screenshots

Light	Dark

Validation

• composer validate --no-check-publish
• interactively verified outline, the opaque reference treatment, and soft across white/zinc/blue backdrops in light and dark mode
• compared rendered screenshot pixels on the base backdrop: exact light fill/border match, exact dark border match, and a one-value dark fill difference

[calebporzio]

@copilot resolve the merge conflicts in this pull request