[client] Add sleep state tracking to handle wakeup/sleep events reliably (#4894)

Adds a new NotifyOSLifecycle RPC and server handler to centralize OS sleep/wake handling, introduces Server.sleepTriggeredDown for coordination, updates client UI to call the new RPC, and adjusts the internal sleep event enum zero-value semantics.

client/server/server.go

@@ -85,6 +85,9 @@ type Server struct {
 	profilesDisabled       bool
 	updateSettingsDisabled bool
 
+	// sleepTriggeredDown holds a state indicated if the sleep handler triggered the last client down
+	sleepTriggeredDown atomic.Bool
+
 	jwtCache *jwtCache
 }