Subtler hover styles on main page, shimmer effects

Remove scaling effect on category card hover on main page since it's quite a lot of movement (distracting). It also doesn't really work on mobile since it scales on tap.

This MR makes the hover effect more subtle using a border, and makes the exam/document list cards match the style with reduced shadows.

Finally, it also adds some shimmer and sparkle effects to some important buttons including Add Exam, Add Document, and Add Answer.