fix: mp-3213-center the switch knob on the track when the label wraps

[infante-jaime]

MP-3213 — Center the switch knob when the label wraps

Fixes https://kiva.atlassian.net/browse/MP-3213

At mobile widths, a KvSwitch label that wraps to multiple lines made the
row taller than the track, and the knob — anchored to the top of the row —
drifted off the track.

Changes

• Anchor the knob to the vertical center (tw-top-1/2 -tw-translate-y-1/2)
  instead of tw-top-0, so it stays centered on the track at any label height.
• Add a wrapped-label example to the Forms/KvSwitch Small story to show the case.
• Add a unit test asserting the centering classes.

Testing

• npm run test — all KvSwitch specs pass, including the new centering test.
• Verified in Storybook with the wrapped-label example at ~320px width.

In the Storybook, we deliberately added a second toggle with a short label so that it needs to be centered by the fix.

[dyersituations]

@infante-jaime please post a screenshot of the fixed switch to the description of the PR.

[roger-in-kiva]

Is there a case with more than 2 lines?

[infante-jaime]

Is there a case with more than 2 lines?

Yes — the fix works regardless of the number of lines. The track and knob are both centered against the same row, so they stay aligned at any label height.

I also verified it with a 10-line label. The story just uses 2 lines to keep the example simple.