feat: Adds battery monitor service, timer, and execution script.

scripts/arch/battery-monitor

@@ -0,0 +1,32 @@
+#!/bin/zsh
+
+# Used with the systemd battery monitor service and timer to alert
+# when the battery has dropped below the threshold and is not currently
+# charging.
+
+THRESHOLD=20 # Notify when below 20%
+NOTIFICATION_FLAG="/run/user/${UID}/user_battery_notified"
+
+function get-battery-percentage() {
+  local battery=$(upower --battery | grep percentage)
+  echo "${battery//[^0-9]/}"
+}
+
+function send-battery-alert() {
+  notify-send -u critical \
+    "Recharge battery!" "Batttery is down to ${1}" \
+    -i battery-caution \
+    -t 30000
+}
+
+battery_percentage=$(get-battery-percentage)
+battery_state=$(upower --battery | grep -E state | awk '{print $2}')
+
+if [[ "$battery_state" == "discharging" && "$battery_percentage" -le "$THRESHOLD" ]]; then
+  if [ ! -f "$NOTIFICATION_FLAG" ]; then
+    send-battery-alert "$battery_percentage"
+    touch "$NOTIFICATION_FLAG"
+  fi
+else
+  rm -f "$NOTIFICATION_FLAG"
+fi