Improve keyboard navigation on boards

What does this MR do and why?

Add keyboard nav to boards

• Treats cards as focusable buttons
• Adds up and down traversal within list
• Adds left and right traversal across lists

Follow ups

• Add support for moving items via keyboard shortcuts

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open a board
2. Tab to focus on a list card
3. Once focused within a list, navigate via arrow keys
4. Open an item using Enter (should be same as clicking the card)
5. Tab still allows focusing inner elements (should not be focus trapped)

Relates #467707 Closes #491629 (closed)