state+components: reimplement 'isDark' as a selector

We now have a consolidated source of truth for whether the UI should us
dark mode to accomodate a light icon color. This is a recoil selector
that derives from iconColorAtom.

In addition, we renamed styleQueryAtom -> iconStyleAtom for consistency.

src/state/atoms.ts

@@ -6,7 +6,7 @@ export const searchQueryAtom = atom<string>({
   default: "",
 });
 
-export const styleQueryAtom = atom<IconStyle>({
+export const iconStyleAtom = atom<IconStyle>({
   key: "styleQueryAtom",
   default: IconStyle.REGULAR,
 });