Feature/Support Multi-Student Selection For Grant Extension Modal

[JoeMacl]

Description

This PR enhances the Staff Grant Extension modal introduced in #948 by adding support for multi-student selection, allowing staff to grant extensions to multiple students simultaneously. This feature is accessible for admins under the units page.

Note: This branch only contains frontend related changes.

Full documentation for this feature can be found here: https://thoth-tech.netlify.app/products/ontrack/projects/staff-grant-extension/design/

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Logged in as both student and admin users
• Confirmed multi-student selection works correctly
• Validated that form prevents submission without selected students
• Checked success and error messages display appropriately
• Tested modal responsiveness across screen sizes

Screenshot

Testing Checklist:

• Tested in latest Chrome
• Tested in latest Safari
• Tested in latest Firefox

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings