webmail: fix parsing search filter "start:<date>" and "end:<date>"

we were only properly parsing values of "<date>T<time>" or just "<time>".
so you could select a date in the form (or type it), but it would be treated as
just a word of text to search for in messages. so it would quietly do the wrong
thing.

Author: mjl-

webmail/webmail.js

@@ -1866,8 +1866,9 @@ const parseSearchDateTime = (s, isstart) => {
 		return d ? d.toJSON() : undefined;
 	}
 	else if (t.length === 1) {
-		if (isNaN(Date.parse(fixDate(t[0])))) {
-			const d = new Date(fixDate(t[0]));
+		const fds = fixDate(t[0]);
+		if (!isNaN(Date.parse(fds))) {
+			const d = new Date(fds);
 			if (!isstart) {
 				d.setDate(d.getDate() + 1);
 			}

webmail/webmail.ts

@@ -486,8 +486,9 @@ const parseSearchDateTime = (s: string, isstart: boolean): string | undefined =>
 		const d = new Date(fixDate(t[0]) + 'T'+t[1])
 		return d ? d.toJSON() : undefined
 	} else if (t.length === 1) {
-		if (isNaN(Date.parse(fixDate(t[0])))) {
-			const d = new Date(fixDate(t[0]))
+		const fds = fixDate(t[0])
+		if (!isNaN(Date.parse(fds))) {
+			const d = new Date(fds)
 			if (!isstart) {
 				d.setDate(d.getDate()+1)
 			}