feat: add customer messaging interface in shop

- Add shop messages API endpoints (/api/v1/shop/messages)
- Add shop messages page routes (/shop/account/messages)
- Add messages.html template for customer account
- Add Messages card to account dashboard with unread badge
- Customers can view/reply to vendor_customer conversations

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

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

app/templates/shop/account/dashboard.html

@@ -71,6 +71,30 @@
                 </div>
             </div>
         </a>
+
+        <!-- Messages Card -->
+        <a href="{{ base_url }}shop/account/messages"
+           class="block bg-white dark:bg-gray-800 rounded-lg shadow hover:shadow-lg transition-shadow p-6 border border-gray-200 dark:border-gray-700"
+           x-data="{ unreadCount: 0 }"
+           x-init="fetch('/api/v1/shop/messages/unread-count').then(r => r.json()).then(d => unreadCount = d.unread_count).catch(() => {})">
+            <div class="flex items-center mb-4">
+                <div class="flex-shrink-0 relative">
+                    <svg class="h-8 w-8 text-primary" style="color: var(--color-primary)" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 12h.01M12 12h.01M16 12h.01M21 12c0 4.418-4.03 8-9 8a9.863 9.863 0 01-4.255-.949L3 20l1.395-3.72C3.512 15.042 3 13.574 3 12c0-4.418 4.03-8 9-8s9 3.582 9 8z" />
+                    </svg>
+                    <span x-show="unreadCount > 0"
+                          class="absolute -top-1 -right-1 inline-flex items-center justify-center px-1.5 py-0.5 text-xs font-bold leading-none text-white bg-red-600 rounded-full"
+                          x-text="unreadCount > 9 ? '9+' : unreadCount"></span>
+                </div>
+                <div class="ml-4">
+                    <h3 class="text-lg font-semibold text-gray-900 dark:text-white">Messages</h3>
+                    <p class="text-sm text-gray-600 dark:text-gray-400">Contact support</p>
+                </div>
+            </div>
+            <div x-show="unreadCount > 0">
+                <p class="text-sm text-primary font-medium" style="color: var(--color-primary)" x-text="unreadCount + ' unread message' + (unreadCount > 1 ? 's' : '')"></p>
+            </div>
+        </a>
     </div>
 
     <!-- Account Summary -->