Improve style for AdBlock warning

[AnandaCampelo]

Description:

This PR updates the adblock detection script to display the warning message at the top of the page, styled to ensure it is clearly visible and centered inside the box.

Fixes #17784

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

[sgiehl]

Checked the changes locally and it now looks like this:

Thanks for your contribution @AnandaCampelo