Improve UserAvatar and SelectUser components

[fritzSF]

Purpose

Improve the Advanced "Add Coverages" modal with better component flexibility and keyboard accessibility.

• Extend UserAvatar with an x-small size option to fit compact UI contexts
• Expose inlineLabel and labelHidden props to SelectUser for more layout flexibility
• Apply these improvements to the Advanced coverages modal UI

What has changed

UserAvatar

• Added x-small to the supported size variants

SelectUser

• Exposed inlineLabel prop — renders the label inline with the select control
• Exposed labelHidden prop — visually hides the label while keeping it accessible

Advanced "Add Coverages" modal

• Updated coverage rows to use the new x-small avatar size
• Override inlineLabel and labelHidden defaults to show the label above the input, consistent with other form fields

Steps to test

1. Go to a Planning item in edit mode
2. Click the (+) button to add a new coverage
3. Select Advanced mode from the dropdown
4. Verify the coverage row layout looks correct (avatar size, label alignment)
5. Add and duplicate coverages using only the keyboard — confirm focus management and tab order work as expected

Checklist

• I reviewed my code
• I tested all affected functionality and made sure it works

Resolves: SDBELGA-1095

[eos87]

Looks good. There is one minor thing though. Belga is using release/2.10 so we would probably need to cherry pick this one into it, test it there and create a separate PR.