Fix keyboard activation when using prefetch: 'click'

[pedroborges]

Keyboard activation (Enter/Space) isn't working for links using prefetch: 'click' because the click event handler prevented default behavior but didn't trigger the mousedown/mouseup events used for prefetching and navigation. Fixes #2274.

Without proper keyboard support, links become unusable for users with motor disabilities, screen reader users, and anyone who navigates with a keyboard.

I fixed this issue by adding:

• keydown/keyup event handlers to mirror the existing mousedown/mouseup behavior
• shouldActivateOnKey() to ensure Space only activates buttons (not links, per accessibility standards)
• tests to verify keyboard activation works correctly

[pascalbaljet]

Thanks!