Introduce Color value object across wallet builders and Pass models

[Korbeil]

Summary

This change replaces ad-hoc color handling with a small, validated Color value object and threads it through shared builder state and all supported wallet platforms.

What changed

• Added Jolicode\WalletKit\Common\Color with factories for RGB components, #rrggbb hex, and rgb(r, g, b) strings, plus rgb() and hex() serializers; invalid input throws InvalidColorException.
• Updated CommonWalletState and wallet builders so background, foreground, and label colors are typed as Color instead of strings.
• Adjusted Apple, Google Wallet (Android), and Samsung pass models and normalizers to consume or emit colors consistently with the new type.
• Removed the internal ColorMapper in favor of the unified Color API.

Testing

Extended or updated normalizer and builder tests; added coverage for Color behavior.