Returned by GET /api/me/settings, and by a PATCH once applied.
The client's useSettings reads it from the session.
Null until the user picks one, so the instance's default style
(GERBEUR_DEFAULT_STYLE) applies, and so a browser's earlier choice can be
carried over on first sign-in.
colorScheme: ColorScheme | null
Null until the user picks one, which reads as "auto".
digestWeekly: boolean
Whether the weekly digest is sent.
digestMonthly: boolean
Whether the monthly recap is sent.