feat: Add DNS configuration check and update subdomain management UI

extensions/Servers/Proxmox/TechnitiumDns.php

@@ -11,6 +11,15 @@ class TechnitiumDns
     private string $baseUrl;
     private string $token;
 
+    /**
+     * Check whether Technitium DNS credentials are configured.
+     */
+    public static function isConfigured(): bool
+    {
+        return !empty(config('services.technitium.url'))
+            && !empty(config('services.technitium.token'));
+    }
+
     public function __construct()
     {
         $this->baseUrl = rtrim((string) config('services.technitium.url', ''), '/');