Fixes UI navigation in engineering power-monitor program (#19940)

When selecting a sensor in the engineering power-monitor program, you
are not able to return to the sensor list without restarting the whole
console.
Even though the issue is known for some time now, i weren't able to find
an issue to reference in the PR.

- Fixes broken UI navigation when trying to return to the sensor list.
- Resets UI navigation state when program gets closed.

code/modules/modular_computers/file_system/programs/engineering/powermonitor.dm

@@ -20,12 +20,10 @@
 
 /datum/computer_file/program/power_monitor/ui_data(mob/user)
 	var/list/data = initial_data()
-
 	var/list/sensors = list()
-	// Focus: If it remains null if no sensor is selected and UI will display sensor list, otherwise it will display sensor reading.
-	var/obj/machinery/power/sensor/focus
+	var/obj/machinery/power/sensor/focus // Placeholder for selected sensor, if one has been selected from the UI list
 
-	// Build list of data from sensor readings.
+	// Prepare list of sensors (see refresh_sensors()) and set focus to sensor based of active_sensor (see ui_act()->setsensor)
 	for(var/obj/machinery/power/sensor/S in grid_sensors)
 		sensors.Add(list(list(
 		"name" = S.name_tag,
@@ -34,12 +32,18 @@
 		if(S.name_tag == active_sensor)
 			focus = S
 
-	data["all_sensors"] = sensors
+	// Prepare return value: data["focus"] will dictate which view is visible (list or focused sensor)
+	data["all_sensors"] = sensors // Represents main menu list of all found sensors
 	if(focus)
-		data["focus"] = focus.return_reading_data()
-
+		data["focus"] = focus.return_reading_data() // Force view to selected sensor, include focused sensors data
+	else
+		data["focus"] = null // Force view to main menu
 	return data
 
+/datum/computer_file/program/power_monitor/kill_program(forced)
+	..()
+	active_sensor = null // Reset UI navigation state
+
 /datum/computer_file/program/power_monitor/proc/has_alarm()
 	for(var/obj/machinery/power/sensor/S in grid_sensors)
 		if(S.check_grid_warning())