fix(ui): give Button a pointer cursor and the secondary variant a hover

[datlechin]

Follow-up to #36. The consent bar's Allow and Decline show an arrow cursor and nothing on hover, so they don't look clickable.

They are the first secondary Buttons to render as <button> rather than <a>. Two gaps meet there:

• Cursor. Tailwind v4's preflight leaves <button> on the default cursor, and the primitive never set one. A link gets the pointer from the browser.
• Hover. The secondary variant had no hover state at all. The link's pointer had been the only feedback it ever gave.

Change

• cursor-pointer on the Button primitive.
• On hover, secondary steps up to border-rule-strong over a bg-muted fill.
• The transition now covers background and border as well as opacity.

The hover fill measures 1.09:1 against the card in light and 1.19:1 in dark, alongside the border going from 7% to 14%. It was checked visually in both themes in headless Chrome.

Every other secondary Button on the site is a link (hero GitHub, the database pages' docs links, the Download page's second architecture) and gains the same hover.

Testing

• AnalyticsConsentTest pins cursor-pointer in the base classes and a hover: on the secondary variant. It fails with the cursor removed.
• tsc --noEmit is clean.
• In headless Chrome, the computed cursor is pointer and the hover background is oklch(0.97 0 0) in light and oklch(0.269 0 0) in dark.