Fix setup/auth flow and harden storage write failures

src/Domain/UserRepository.php

@@ -69,7 +69,13 @@ final class UserRepository
 
         $this->write(['users' => $users]);
 
-        return $this->find($normalized) ?? [];
+        $createdUser = $this->find($normalized);
+
+        if ($createdUser === null) {
+            throw new RuntimeException('Der Account konnte nicht gespeichert werden.');
+        }
+
+        return $createdUser;
     }
 
     public function changePassword(string $username, string $password): void
@@ -94,9 +100,14 @@ final class UserRepository
             mkdir(dirname($this->path), 0775, true);
         }
 
-        file_put_contents(
+        $bytes = file_put_contents(
             $this->path,
-            json_encode($payload, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES)
+            json_encode($payload, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES),
+            LOCK_EX
         );
+
+        if ($bytes === false) {
+            throw new RuntimeException('Die Benutzerdatei konnte nicht geschrieben werden. Bitte prüfe die Schreibrechte von storage/system.');
+        }
     }
 }