fix: update vendor messages template to use correct page_header API

Changed from old buttons=[] parameter to action_label/action_onclick
parameters which are the correct API for page_header macro.

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

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

app/templates/vendor/messages.html

@@ -8,9 +8,7 @@
 {% block alpine_data %}vendorMessages({{ conversation_id or 'null' }}){% endblock %}
 
 {% block content %}
-{{ page_header('Messages', buttons=[
-    {'text': 'New Conversation', 'icon': 'plus', 'click': 'showComposeModal = true', 'primary': True}
-]) }}
+{{ page_header('Messages', action_label='New Conversation', action_icon='plus', action_onclick='showComposeModal = true') }}
 
 {{ loading_state('Loading conversations...') }}