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/templates/footer.php

@@ -12,21 +12,5 @@
         | Session: <?php echo session_id() !== '' ? 'aktiv' : 'inaktiv'; ?>
     </p>
 </footer>
-
-<?php if (!empty($_SESSION['success']) || !empty($_SESSION['error'])): ?>
-    <script>
-        (function () {
-            const success = <?php echo json_encode($_SESSION['success'] ?? '', JSON_UNESCAPED_UNICODE); ?>;
-            const error = <?php echo json_encode($_SESSION['error'] ?? '', JSON_UNESCAPED_UNICODE); ?>;
-            if (success) {
-                alert(success);
-            }
-            if (error) {
-                alert(error);
-            }
-        })();
-    </script>
-    <?php unset($_SESSION['success'], $_SESSION['error']); ?>
-<?php endif; ?>
 </body>
 </html>