fix: send form date values as YYYY-MM-DD strings to epteam api

public/typo3conf/ext/ep_theme/Classes/Form/AbstractMappedFinisher.php

@@ -34,6 +34,12 @@ abstract class AbstractMappedFinisher extends AbstractFinisher
         return $mappings;
     }
 
+    /**
+     * The mapped values, keyed by the remote name the receiving side expects. Values pass through
+     * normalizeValue() just like the labeled ones do, so a date element arrives as a Y-m-d string
+     * instead of a serialized DateTime. A value that does not survive normalization keeps its key:
+     * the remote names make up the request shape and dropping one would change it.
+     */
     protected function buildPayload(): array
     {
         $values = [];
@@ -44,7 +50,7 @@ abstract class AbstractMappedFinisher extends AbstractFinisher
                 continue;
             }
 
-            $values[$remote] = $formValues[$local];
+            $values[$remote] = $this->normalizeValue($formValues[$local], $local);
         }
 
         return $values;