Better probe downtime tracking and email notifications

[jimaek]

• Add more states like "Reconnecting" if it went down briefly, "Offline" with red dot in case of true long downtime, "Updating" if the probe is doing something we know causes downtime
• Add a tooltip with time/duration, e.g. "Offline since ..."
• Update backend logic to ensure downtime is reliable
• Email the user if a probe is down for more than 24 hours.
• Add option in settings to disable such notifications