Improve dataset view cards

[Atmosfearful]

Goal

Changes to cards so that UX is more consistent-- card sizing, button placement, and badge placement are more predictable now.
Less visual clutter overall.

Closes #650

Screenshots

Before

After

What I changed and why

• Add placeholder images to all cards - now buttons and text don't jump around (improves ux)
• Switch to a clickable Link Card instead of the big purple Button, to reduce purple visual clutter / repetitiveness
• Move badges to the top left, and Settings link to the top right, to use space more effectively and free up more room for title/description
• Remove the redundant Letter Circle decoration because it was just repeating the first letter of the Title which is already visible
• Remove the Show More because it is not compatible with card links - a link-within-link would be non-compliant html (minor tradeoff, worth it IMO)
• Increase tap area of the Settings icon button so it's more compliant with a11y standards (40px)
• Fix tab navigation, ensure the focused element always has an outline

How I convinced myself this is right

A mix of subjective preference and objective a11y/usability considerations

What I'm not doing here

Changing behavior

LLM use disclosure

GPT-6 Luna with OpenCode to tweak html/tailwind classes

[conservationtimothy]

OK with this but I'd like @rudokemper's product eyes on this ; )

[rudokemper]

Thanks! Looks great to me. And in the future for any Explorer UI/UX please also feel free to tag in @nicopace or @maypontes to give feedback from the programs side.