fix(import): preserve background and border resets

[tommy230]

Summary

Imported styles that use common resets such as background: none or border: none can be expanded into invalid colour declarations like background-color: none. Browsers discard those colour declarations, which can allow an earlier theme colour to remain visible instead of applying the authored reset.

Normalize none to initial only when it reaches a colour longhand. Valid uses on image and style properties remain unchanged, as do custom properties and real authored colours.

Verification

• bun run build
• bun test (6,622 passed)
• bun run lint
• Focused shorthand and bundle-budget tests
• Docker/deployment check, not relevant to this importer-only change

Checklist

• Tests cover behavior changes.
• Docs were reviewed; no public behavior or configuration documentation changes are needed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.