feat(VLE): Separate activities visually in step view

[breity]

Changes

• In the student step view for units, separated each activity visually with a card background and space between other activities.
• Cleaned up activity (component) styles and elements to better match the new separated layout.
• Styled activity titles and added activity type icons when title is present.

Test

• Make sure all activity types operate and function as before in the student view.
• Make sure grading view for each activity type functions properly, as before.