Custom Fields Now Respect Plan Limits

Custom fields and agreement features now properly respect plan limits across the platform.

What changed:

• Plan-based availability — Custom fields are now controlled by your plan tier
• Clearer messaging — UI now shows "your plan" instead of specific plan names for consistent messaging
• Better backward compatibility — Improved handling of null values in plan limits

This update ensures feature availability is consistent across all plan tiers and makes it clearer what features are included in your plan.