Add warning that mail may be removed when "Sync messages from" is decreased

Feature request

When "Account Settings" -> "Fetching mail" -> "Sync up to" interval is changed from a higher value to a lower one (including changing from "any time" to anything else), already downloaded messages or headers will be removed from the client (see here). This is absolutely not evident because "sync" can imply that it limits only the time range from which new messages will be downloaded, not that is also affects the messages which are already downloaded.

I'd like to request a warning message which would be displayed every time the user decreases the "Sync messages from" value. Something like "Warning: this will remove previously downloaded messages from the phone that fall outside this range. Messages on the server are not affected."

Environment

K-9 Mail version: 5.208

Start at Account Settings → Fetching mail → Sync up to and trace how changes to the interval are handled. Add a warning when the value is decreased, including from “any time,” explaining that previously downloaded messages outside the range are removed while server messages are unaffected.