Activate/deactivate Cloud Playout channels with notifications

Channel lifecycle actions have been overhauled.

• Activating or deactivating a channel now triggers a notification so you have a clear record of every state change.
• Activation and deactivation timers are exposed so operators know exactly when a channel will go live.
• Notifications for channel state changes are now separated from generic schedule notifications, making it easier to spot critical events.
• Activate-button behavior was fixed for the schedule channel where the action previously got stuck in a loading state.

The flow respects offline detection — pressing Activate without internet shows a clear error instead of silently failing.