[SandObserver]

Summary

• Parse a CSS color with two seeded fillStyle reads, so _cssToHex returns null for an unparseable value instead of the seeded black.
• Add normalizeColorInput, which supplies a missing leading # for a bare 3 to 8 digit hex code and reports whether the value parses at all.
• Use it in the color control's code row and in the appearance background color field, and show a toast when the value is not a color.
• Add toast.colorInvalid to all six catalogs.

Testing
All green. Added 5 frontend tests for the parser and the normaliser, and updated the case that pinned black for an unreadable value.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.