Add thank you page and fix webhook stock update

- Create thank you page template with bilingual content (FR/EN)
- Add automatic redirect to /thanks after successful payment
- Fix webhook: move kirby()->impersonate() before product update
- Add error handling in webhook with try-catch
- Pass order token to thank you page for future enhancements
- Update thank you page styling for better UX

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/templates/thanks.php

@@ -0,0 +1,28 @@
+<?php snippet('header', ['title' => $page->title()]) ?>
+
+    <main>
+        <section class="thanks-page">
+            <div class="thanks-content">
+                <h1><?= $page->title()->html() ?></h1>
+
+                <?php if($page->text()->isNotEmpty()): ?>
+                    <div class="thanks-message">
+                        <?= $page->text()->kt() ?>
+                    </div>
+                <?php endif ?>
+
+                <div class="thanks-actions">
+                    <a href="<?= $site->homePage()->url() ?>" class="btn__default">
+                        <span class="icon">
+                            <svg clip-rule="evenodd" fill-rule="evenodd" stroke-linejoin="round" stroke-miterlimit="2" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
+                                <path d="m9.474 5.209s-4.501 4.505-6.254 6.259c-.147.146-.22.338-.22.53s.073.384.22.53c1.752 1.754 6.252 6.257 6.252 6.257.145.145.336.217.527.217.191 0 .383-.072.53-.221.293-.293.294-.766.004-1.057l-4.976-4.976h14.692c.414 0 .75-.336.75-.75s-.336-.75-.75-.75h-14.692l4.978-4.979c.289-.289.287-.761-.006-1.054-.147-.147-.339-.221-.53-.221-.191 0-.38.071-.525.215z" fill-rule="nonzero" />
+                            </svg>
+                        </span>
+                        <div class="txt"><?= t('backToShop', 'Retour à la boutique') ?></div>
+                    </a>
+                </div>
+            </div>
+        </section>
+    </main>
+
+<?php snippet('footer') ?>