notification service initial

system/systemd/coredesktop-update-check.service

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+[Unit]
+Description=Notify when a new version of CoreDesktop is available
+# Notifications need a desktop session to show in
+ConditionEnvironment=|WAYLAND_DISPLAY
+ConditionEnvironment=|DISPLAY
+
+[Service]
+# Not oneshot: the script waits, without a time limit, for the notification
+# to be clicked or dismissed
+Type=simple
+ExecStart=%h/.local/share/coredesktop/system/update-check

system/systemd/coredesktop-update-check.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Check for a new version of CoreDesktop every few hours
+
+[Timer]
+OnStartupSec=5min
+# Counted from when the last check finished, so a notification left open for
+# hours doesn't have another check queued up behind it
+OnUnitInactiveSec=4h
+
+[Install]
+WantedBy=timers.target