Fix setup/auth flow and harden storage write failures

src/Support/Auth.php

@@ -10,7 +10,13 @@ final class Auth
 
     public function check(): bool
     {
-        return isset($_SESSION['user']) && is_array($_SESSION['user']);
+        if (!isset($_SESSION['user']) || !is_array($_SESSION['user'])) {
+            return false;
+        }
+
+        $username = $_SESSION['user']['username'] ?? null;
+
+        return is_string($username) && $username !== '';
     }
 
     public function user(): ?array
@@ -37,6 +43,10 @@ final class Auth
 
     public function login(array $user): void
     {
+        if (!isset($user['username']) || !is_string($user['username']) || $user['username'] === '') {
+            throw new RuntimeException('Der Benutzer konnte nicht angemeldet werden.');
+        }
+
         session_regenerate_id(true);
 
         $_SESSION['user'] = [
@@ -51,4 +61,3 @@ final class Auth
         session_regenerate_id(true);
     }
 }
-