feat: add demo entries with images + fix form upload action

- New demo entry: Arashiyama with single hero image (bamboo.jpg)
- New demo entry: Gyeongbokgung with four gallery images (palace-gate,
  throne-hall, hanok-rooftops, bugaksan)
- post-form.md: add upload: true to process block so filepond photo
  uploads are handled after page creation; simplify list-of-maps to
  flat map (Symfony YAML preserves insertion order)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pages/02.post/post-form.md

@@ -94,10 +94,8 @@ form:
             classes: btn-post
 
     process:
-        -
-            add_page: true
-        -
-            message: 'Entry posted successfully!'
-        -
-            reset: true
+        add_page: true
+        upload: true
+        message: 'Entry posted successfully!'
+        reset: true
 ---