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/blueprints/pages/thanks.yml

@@ -0,0 +1,16 @@
+title:
+  en: Thank you
+  fr: Merci
+icon: check
+
+tabs:
+  content:
+    label:
+      en: Content
+      fr: Contenu
+    fields:
+      text:
+        label:
+          en: Message
+          fr: Message
+        type: writer