Request Flow State Machine

Write uiStateFor(event) — a fetch's UI state machine. States: "idle", "loading", "success", "empty", "error". Transitions: ("fetch" => "loading"), ("resolve-with-data" => "success"), ("resolve-empty" => "empty"), ("reject" => "error"), ("reset" => "idle"). Invalid transitions (e.g. "success" from "idle") return "idle-state-invalid".

Difficulty: intermediate