Restore support for Mastodon profile short-form URLs

- Running these URLs through WebFinger first allows to retrieve the canonical ActivityPub profile URL

src/Model/APContact.php

@@ -157,7 +157,10 @@ class APContact
 
 		$apcontact = [];
 
-		$webfinger = empty(parse_url($url, PHP_URL_SCHEME));
+		// Mastodon profile short-form URL https://domain.tld/@user doesn't return AP data when queried
+		// with HTTPSignature::fetchRaw, but returns the correct data when provided to WebFinger
+		// @see https://github.com/friendica/friendica/issues/13359
+		$webfinger = empty(parse_url($url, PHP_URL_SCHEME)) || strpos($url, '@') !== false;
 		if ($webfinger) {
 			$apcontact = self::fetchWebfingerData($url);
 			if (empty($apcontact['url'])) {