fix: throw an error, if there is no ADMINFORTH_SECRET variable in environment

Author: yaroslav8765

adminforth/modules/configValidator.ts

@@ -1255,6 +1255,14 @@ export default class ConfigValidator implements IConfigValidator {
       throw new Error(`Invalid AdminForth config: ${errors.join(', ')}`);
     }
 
+    const adminforthSecret = process.env.ADMINFORTH_SECRET;
+    if (!adminforthSecret) {
+      throw new Error(`ADMINFORTH_SECRET environment not set
+        Please set ADMINFORTH_SECRET environment variable to a random string to secure your admin panel.
+        ADMINFORTH_SECRET variable is used to sign JWT tokens
+      `);
+    }
+
     this.adminforth.config = newConfig as AdminForthConfig;
   }