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>

content/thanks/thanks.fr.txt

@@ -0,0 +1,9 @@
+Title: Merci pour votre commande
+
+----
+
+Text:
+
+Votre commande a été confirmée ! Vous allez recevoir un email de confirmation sous peu.
+
+Merci pour votre achat.

content/thanks/thanks.txt

@@ -0,0 +1,9 @@
+Title: Thank you for your order
+
+----
+
+Text:
+
+Your order has been confirmed! You will receive a confirmation email shortly.
+
+Thank you for your purchase.