feat: add vendor messaging interface

- Add vendor messages API endpoints (/api/v1/vendor/messages)
- Add vendor messages page route (/vendor/{code}/messages)
- Add messages.html template for vendor portal
- Add messages.js Alpine component
- Add Messages link to vendor sidebar under Sales section

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

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

app/templates/vendor/partials/sidebar.html

@@ -117,6 +117,17 @@ Follows same pattern as admin sidebar
                     <span class="ml-4">Customers</span>
                 </a>
             </li>
+            <li class="relative px-6 py-3">
+                <span x-show="currentPage === 'messages'"
+                      class="absolute inset-y-0 left-0 w-1 bg-purple-600 rounded-tr-lg rounded-br-lg"
+                      aria-hidden="true"></span>
+                <a class="inline-flex items-center w-full text-sm font-semibold transition-colors duration-150 hover:text-gray-800 dark:hover:text-gray-200"
+                   :class="currentPage === 'messages' ? 'text-gray-800 dark:text-gray-100' : ''"
+                   :href="`/vendor/${vendorCode}/messages`">
+                    <span x-html="$icon('chat-bubble-left-right', 'w-5 h-5')"></span>
+                    <span class="ml-4">Messages</span>
+                </a>
+            </li>
         </ul>
 
         <!-- Settings Section -->