feat: implement package 1 session and validation feedback

- add session validation_errors bootstrap initialization

- render global flash + validation messages in header

- remove footer alert-based flash handling

- persist structured validation errors across save handlers

- mark NEXT_STEPS package 1 tasks as done

app/modules/racks/save.php

@@ -40,6 +40,7 @@ if ($heightHe < 1) {
 // Falls Fehler: zurück zum Edit-Formular
 if (!empty($errors)) {
     $_SESSION['error'] = implode(', ', $errors);
+    $_SESSION['validation_errors'] = $errors;
     $redirectUrl = $rackId ? "?module=racks&action=edit&id=$rackId" : "?module=racks&action=edit";
     header("Location: $redirectUrl");
     exit;