refactor(client): derive option groups from the shared dictionary map

Plain group reads now select from the single all-groups dictionary
instead of firing one request per group, so option-backed fields
resolve synchronously once the map is warm. Location groups and
parent/search filtered reads keep their dedicated endpoint - the city
group is too large for the dictionary and location filtering is
server-side. Both layout routes warm the dictionary through a router
loader that degrades gracefully on failure, eliminating the raw-value
flash on cold-cache first paint.