Localization: "Add Custom Locale" tab has no icon, while the sibling "Add Locale" tab has one

Steps to reproduce:

1. Create a new project
2. Install the Localization package (com.unity.localization)
3. Open Edit > Project Settings > Localization and click "Create" to make the settings asset
4. Click "Add Locale" and note the icon on its tab
5. Click "Add Custom Locale" and compare its tab with step 4

Actual results: The "Add Custom Locale" tab shows a label with no icon, while "Add Locale" shows one. The two windows do not read as belonging to the same feature

Expected results: The "Add Custom Locale" tab carries an icon consistent with its sibling window

Reproducible with versions: 1.5.12 (6000.5.7f1, 6000.6.0f1, 6000.7.0a6)

Tested on (OS): MacBook Pro - Tahoe 26.6.2