webmail: don't who an age of "-<1min", drop the -

if a browser is ahead just a few seconds, we would show "-<1min", not great.
just show "<1min" in that case. we'll still show negative age if drift is more
than 1 minute, which seems like a good hint to get time fixed on either client
or server.

webmail/webmail.ts

@@ -1939,6 +1939,8 @@ const newMsgitemView = (mi: api.MessageItem, msglistView: MsglistView, otherMail
 			if (t < 60) {
 				s = '<1min'
 				nextSecs = 60-t
+				// Prevent showing '-<1min' when browser and server have relatively small time drift of max 1 minute.
+				negative = ''
 			}
 
 			dom._kids(r, negative+s)