Fixed some static analysis errors

app/src/Exceptions/InvalidConfiguration.php

@@ -4,7 +4,7 @@ namespace App\Exceptions;
 
 use RuntimeException;
 
-class InvalidConfiguration extends RuntimeException
+final class InvalidConfiguration extends RuntimeException
 {
     /** Create an exception from a configuration option and value. */
     public static function fromConfig(string $option, $value): self

app/src/HiddenFiles.php

@@ -27,6 +27,6 @@ class HiddenFiles extends Collection
             $items = array_merge($items, $config->get('app_files'));
         }
 
-        return new static(array_unique($items));
+        return new self(array_unique($items));
     }
 }

psalm-baseline.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<files psalm-version="3.12.1@9b860214d58c48b5cbe99bdb17914d0eb723c9cd">
+<files psalm-version="3.15@de6e7f324f44dde540ebe7ebd4eb481b97c86f30">
   <file src="app/src/Controllers/DirectoryController.php">
     <PossiblyInvalidMethodCall occurrences="1">
       <code>current</code>