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/devices/save.php

@@ -57,6 +57,7 @@ if ($rackHeightHe < 1) {
 // Falls Fehler: zurück zum Edit-Formular
 if (!empty($errors)) {
     $_SESSION['error'] = implode(', ', $errors);
+    $_SESSION['validation_errors'] = $errors;
     $redirectUrl = $deviceId ? "?module=devices&action=edit&id=$deviceId" : "?module=devices&action=edit";
     header("Location: $redirectUrl");
     exit;