Support managing trait/option availability across variants from the trait itself

Summary

Currently, when adding a new trait or trait option, you have to individually visit each variant to enable it. This is tedious, especially for registries with many variants. We should support a two-way experience where you can also manage variant availability directly from the trait or trait option.

Current Behavior

1. Create a new trait or trait option.
2. Navigate to each variant individually.
3. Enable the trait/option on each variant one by one.

This is repetitive and error-prone, especially as the number of variants grows.

Desired Behavior

• When viewing/editing a trait or trait option, provide easy access to control which variants it is available on.
• This could be a checklist of variants, a bulk toggle, or similar UI that lets you enable/disable the trait across all variants in one place.
• The existing per-variant configuration should continue to work as well (two-way management).

Benefits

• Dramatically reduces the number of clicks needed when adding new traits or options.
• Reduces the chance of forgetting to enable a trait on a specific variant.
• Makes the admin experience much smoother for registries with many variants.

No files, tests, or entry points are identified. Start by locating the trait, trait-option, and variant admin screens and their existing per-variant availability flow; define the two-way behavior and verify that changes made from either screen stay consistent.