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`.
Moving the sidebar highlight (keyboard or hover) now shows that session in
the content pane without opening it: its own tab comes forward, or a
throwaway tab carries it. The layout, the saved layout and the daemon's
focus report are all left untouched, and the keyboard stays in the list.
Contract in docs/PREVIEW.md.
Miguel Palhas