FlagService
Server entry Script. Creates remotes, initializes FlagRegistry, hooks prompts,
and handles RequestSetNation with FlagAuth checks.
Remotes under ReplicatedStorage.NationFlagSystem.Remotes:
OpenPicker(server → client):poleId,currentNationIdRequestSetNation(client → server):poleId,nationIdNotify(server → client): toastmessage