feat: slide the highlight when switching modes

[emakko]

What

The Swipe · Remove duplicates switch in the playlist picker now animates. The white highlight slides to the chosen mode and resizes to fit it, and the label colours fade, over 0.25 s.

• PlaylistPicker.tsx: a .seg-pill element sits behind the buttons. A layout effect measures the active button when the mode changes, and the pill moves there with transform and width.
• styles.css: the buttons no longer draw their own highlight; the pill does. The animation is turned off under prefers-reduced-motion.

Notes for review

• The pill first renders already in place, so the page doesn't animate on load. It only animates when you switch.
• Buttons stay native <button>s with aria-pressed. The pill is aria-hidden.

Testing

• npm test: 129 passed. npm run build: succeeds.
• Checked in the browser on a temporary page that rendered the picker with fake data, not committed. At 100 ms the pill was mid-slide. It then lined up with the chosen button, including the extra width from the bold label, in both directions.

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