CooldownCountdownChip class

Adapter over CountdownChip for a keyed cooldown read from PersistentCooldownService — takes plain remaining data rather than reaching into the service directly (same convention as EnergyBar/ LevelSelectGrid: widgets take data, callers own the GetX service).

Renders nothing while remaining is zero (cooldown ready — nothing to count down). remaining is only converted into a fixed DateTime target once, at mount and whenever it actually changes value — a parent rebuild that passes the same remaining again (e.g. an unrelated Obx tick) does not reset the underlying countdown.