Cannot link a slot property across existing variants — must rebuild from scratch

Summary

When adding a slot to a component set that already has multiple variants, there is no way to link that slot property to matching layers in the other variants. The only way to get a unified slot across variants is to add the slot to one variant first, then duplicate to create the others. This means any existing component with multiple variants must be rebuilt from scratch to add a shared slot.

Steps to reproduce

1. Create a component with two or more variants.
2. In one variant, add a slot property to a layer.
3. In another variant, select the matching layer (same name, same hierarchy).
4. There is no option to assign it to the existing slot property.
5. Creating a new slot property on that layer produces a duplicate — two separate properties with a conflicting variants warning.

Working path (for comparison)

1. Create a component with one variant.
2. Add a slot property to a layer.
3. Add a new variant (duplicate).
4. The slot property carries over as a single unified property showing "2 items".

Expected behavior

When applying a slot to a layer in one variant, there should be a way to assign matching layers in other variants to the same slot property — the same way boolean and text properties can be linked across variants.

Requested fix

Allow assigning an existing slot property to layers in other variants, either through multi-edit or a dropdown when applying the property.