fix: allow querying the root DNS zone (#6602)

Co-authored-by: Frank Elsinga <frank@elsinga.de>

src/pages/EditMonitor.vue

@@ -2083,8 +2083,9 @@ message HealthCheckResponse {
                     return false;
                 }
 
-                // Wildcard is allowed only for DNS
-                if (!isFQDN(hostname, {
+                // Root zone "." is valid for DNS but not recognized by isFQDN
+                const isRootZone = this.monitor.type === "dns" && hostname === ".";
+                if (!isRootZone && !isFQDN(hostname, {
                     allow_wildcard: this.monitor.type === "dns",
                     require_tld: false,
                     allow_underscores: true,