Dark theme (any selection).
Light theme, selection unknown (mid-slate fallback).
Light theme, tab selected (white). Unused by pickTabIconUrl today: every current host keeps
the active tab's header on a plain light background (never a dark/tinted one), so a white icon
would be invisible there. Kept in the type for a future host that does give the selected tab a
dark background and needs a contrasting icon.
Light theme, tab not selected (near-black). Also used for the selected state — see above.
The four tab-icon variants, as static asset URLs (e.g.
papi-extension://URLs).