A preview was distinguished only by the chip and a dashed ring, both easy to
miss, and the sidebar's hover reused the neutral surface — so a hovered row
said nothing about what the content pane was showing.
Adds a `--preview` token family on a hue of its own, deliberately not the
accent: the accent means "this is the one you picked", which a preview is not.
The previewed pane's titlebar, the chip, the throwaway tab, the previewed
sidebar row and hover all carry it. Palettes whose accent is already violet
(periwinkle, orchid) move the hue via `--preview-h`.
Miguel Palhas
7a3287f231