feat: add admin messaging interface

- Add admin messages API endpoints (/api/v1/admin/messages)
- Add admin messages page route (/admin/messages)
- Add messages.html template with split-panel conversation view
- Add messages.js Alpine component for messaging UI
- Add Messages link to admin sidebar under Platform Administration
- Add message icon with unread badge to admin header
- Update init-alpine.js with headerMessages component

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

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

app/templates/admin/partials/sidebar.html

@@ -72,6 +72,7 @@
         {{ menu_item('companies', '/admin/companies', 'office-building', 'Companies') }}
         {{ menu_item('vendors', '/admin/vendors', 'shopping-bag', 'Vendors') }}
         {{ menu_item('users', '/admin/users', 'users', 'Users') }}
+        {{ menu_item('messages', '/admin/messages', 'chat-bubble-left-right', 'Messages') }}
     {% endcall %}
 
     <!-- Vendor Operations Section -->