Support wildcard topic in mail permissions

topic: null in a permission entry now matches any topic, allowing
broad grants without specifying a specific topic. set-mail-permission
topic param is now optional; omitting it stores null (wildcard).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/mail_perms.mjs

@@ -23,7 +23,7 @@ export function load_mail_perms(file_path) {
 	}
 
 	function check(user, to, topic) {
-		return allowed.some(e => e.user === user && e.to === to && e.topic === topic);
+		return allowed.some(e => e.user === user && e.to === to && (e.topic === topic || e.topic === null));
 	}
 
 	function add(user, to, topic) {