fix(desktop): slide the lyric sheet on the macOS spring

[aodjo]

The scrolling lyrics reached 96% of their travel a quarter of the way through
the time they were given, so a line change read as a jump rather than a
glide, and the cascade delay tore the rows apart to 42px gaps before snapping
them back to 29px.

The spring constants did not match SwiftUI's spring(duration:bounce:): that
spring's duration is one period of the underlying oscillator, so the
normalised frequency is a full turn of the circle rather than 11.5, and the
damping ratio is one minus the bounce.

Measured on a headless render of the view, frames every 40ms:

	before	after
travel complete	~180ms	~470ms
widest row gap mid-slide	42px	37px

Also here: the highlight follows the line being sung rather than the focused
row, so an instrumental gap dims the sheet the way macOS does instead of
leaving a line lit; the crossfade eases at both ends; and a slide draws the
rows it is travelling from, so a seek no longer pops them in.

https://claude.ai/code/session_0164R7Zuagx737iKYbuuSjnA

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