Refs #23557 - Make 2FA modal action buttons always visible

[tzi]

Description:

Refs #23557

In some modal with a lot of content, or with small devices, the action buttons are not visible by default and the user could be stuck if they didn’t think to scroll.

Two actions in this PR:

• Making the modal footer always visible
• Add a vertical scroll shadow to help the user to see scrollable content

Here some screenshots:

screen	before	after
desktop
mobile

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed