ui-carousel: Fix unnecessary scroll of carousel items into view

[microbit-robert]

Adds a story for a carousel of project cards with actions used to reproduce the bug and test the fix.

Closes #154

[cloudflare-workers-and-pages]

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[microbit-matt-hillsdon]

So we still move a full card width for the sake of a pixel off screen which can feel a bit weird when your click target was fully visible. Say in a two-cards-minus-one-pixel width setup, RH edge just off screen click clearly visible and 100% on-screen button. Maybe the click target should stop propagation (untested) ? Let's chat briefly about this tomorrow.

[microbit-matt-hillsdon]

So we still move a full card width for the sake of a pixel off screen which can feel a bit weird when your click target was fully visible. Say in a two-cards-minus-one-pixel width setup, RH edge just off screen click clearly visible and 100% on-screen button. Maybe the click target should stop propagation (untested) ? Let's chat briefly about this tomorrow.

Or as Claude suggests:

The right distinction is pointer versus keyboard, not visible versus not. If the user clicked or tapped something, it was on screen. Focus-follows-slide only needs to run when focus arrived by keyboard or a screen reader. react-aria tracks this globally and the repo already uses it, for example useFocusVisible in ComboBox. The carousel handler could early-return when getInteractionModality() is "pointer", or equivalently when isFocusVisible() is false. React-aria's press handling focuses the button on pointerdown, after the modality has been set to pointer, so the ordering works. The visibility check would stay as a secondary guard for keyboard focus.

[microbit-matt-hillsdon]

Added the modality fix. Couldn't use react-aria's notion as it's React state which can be updated too late for the carousel but easy enough to track by hand.