'Select all option for Relations'

**Describe the bug**
I have defined only one relation type in the Labeling Setup.
When annotating, I must manually click each pair of regions and select the same relation type every time.

This becomes extremely repetitive when annotating many widget ↔ label mappings on a single page.

There is no “Select All” or “Apply to all” option to automatically assign the only available relation type to multiple relations at once.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment (please complete the following information):**
- Label Studio Version 1.21.0