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/header.php

@@ -64,4 +64,60 @@
     </nav>
 </header>
 
+<?php
+$flashMessages = [];
+
+$successMessage = trim((string)($_SESSION['success'] ?? ''));
+if ($successMessage !== '') {
+    $flashMessages[] = [
+        'type' => 'success',
+        'text' => $successMessage,
+    ];
+}
+
+$errorMessage = trim((string)($_SESSION['error'] ?? ''));
+if ($errorMessage !== '') {
+    $flashMessages[] = [
+        'type' => 'error',
+        'text' => $errorMessage,
+    ];
+}
+
+$validationErrors = $_SESSION['validation_errors'] ?? [];
+if (!is_array($validationErrors)) {
+    $validationErrors = [];
+}
+$validationErrors = array_values(array_filter(array_map(static function ($entry) {
+    return trim((string)$entry);
+}, $validationErrors), static function ($entry) {
+    return $entry !== '';
+}));
+
+if (!empty($validationErrors)) {
+    $flashMessages[] = [
+        'type' => 'error',
+        'text' => 'Bitte pruefe die Eingaben:',
+        'details' => $validationErrors,
+    ];
+}
+
+unset($_SESSION['success'], $_SESSION['error'], $_SESSION['validation_errors']);
+?>
+
 <main>
+    <?php if (!empty($flashMessages)): ?>
+        <section class="flash-stack" aria-live="polite">
+            <?php foreach ($flashMessages as $message): ?>
+                <article class="flash-message flash-message--<?php echo htmlspecialchars((string)$message['type']); ?>">
+                    <p class="flash-message__text"><?php echo htmlspecialchars((string)($message['text'] ?? '')); ?></p>
+                    <?php if (!empty($message['details']) && is_array($message['details'])): ?>
+                        <ul class="flash-message__list">
+                            <?php foreach ($message['details'] as $detail): ?>
+                                <li><?php echo htmlspecialchars((string)$detail); ?></li>
+                            <?php endforeach; ?>
+                        </ul>
+                    <?php endif; ?>
+                </article>
+            <?php endforeach; ?>
+        </section>
+    <?php endif; ?>