feat(admin): add Platform Administration section to sidebar

Reorganized sidebar navigation:
- Added "Platform Administration" section header below Dashboard
- Grouped Companies, Vendors, Users, Customers, Marketplace under it
- Added new Customers page (empty placeholder for future)
- Changed Marketplace icon from shopping-bag to globe
- Renamed "Marketplace Import" to "Marketplace" for brevity

New files:
- app/templates/admin/customers.html
- Route: GET /admin/customers

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

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

app/templates/admin/partials/sidebar.html

@@ -18,8 +18,14 @@
             </li>
         </ul>
 
-        <!-- Main Navigation -->
-        <ul>
+        <!-- Platform Administration Section -->
+        <div class="px-6 my-6">
+            <hr class="border-gray-200 dark:border-gray-700" />
+        </div>
+        <p class="px-6 text-xs font-semibold text-gray-600 dark:text-gray-400 uppercase tracking-wider">
+            Platform Administration
+        </p>
+        <ul class="mt-3">
             <!-- Companies -->
             <li class="relative px-6 py-3">
                 <span x-show="currentPage === 'companies'" class="absolute inset-y-0 left-0 w-1 bg-purple-600 rounded-tr-lg rounded-br-lg" aria-hidden="true"></span>
@@ -53,14 +59,25 @@
                 </a>
             </li>
 
-            <!-- Marketplace Import -->
+            <!-- Customers -->
+            <li class="relative px-6 py-3">
+                <span x-show="currentPage === 'customers'" 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 === 'customers' ? 'text-gray-800 dark:text-gray-100' : ''"
+                   href="/admin/customers">
+                    <span x-html="$icon('user-group')"></span>
+                    <span class="ml-4">Customers</span>
+                </a>
+            </li>
+
+            <!-- Marketplace -->
             <li class="relative px-6 py-3">
                 <span x-show="currentPage === 'marketplace'" class="absolute inset-y-0 left-0 w-1 bg-purple-600 rounded-tr-lg rounded-br-lg" aria-hidden="true"></span>
                 <a class="inline-flex items-center w-full text-sm font-semibold transition-colors duration-150 hover:text-gray-800 dark:hover:text-gray-200"
                    :class="currentPage === 'marketplace' ? 'text-gray-800 dark:text-gray-100' : ''"
                    href="/admin/marketplace">
-                    <span x-html="$icon('shopping-bag')"></span>
-                    <span class="ml-4">Marketplace Import</span>
+                    <span x-html="$icon('globe')"></span>
+                    <span class="ml-4">Marketplace</span>
                 </a>
             </li>
         </ul>
@@ -247,8 +264,14 @@
             </li>
         </ul>
 
-        <!-- Main Navigation -->
-        <ul>
+        <!-- Platform Administration Section -->
+        <div class="px-6 my-6">
+            <hr class="border-gray-200 dark:border-gray-700" />
+        </div>
+        <p class="px-6 text-xs font-semibold text-gray-600 dark:text-gray-400 uppercase tracking-wider">
+            Platform Administration
+        </p>
+        <ul class="mt-3">
             <!-- Companies -->
             <li class="relative px-6 py-3">
                 <span x-show="currentPage === 'companies'" class="absolute inset-y-0 left-0 w-1 bg-purple-600 rounded-tr-lg rounded-br-lg" aria-hidden="true"></span>
@@ -282,14 +305,25 @@
                 </a>
             </li>
 
-            <!-- Marketplace Import -->
+            <!-- Customers -->
+            <li class="relative px-6 py-3">
+                <span x-show="currentPage === 'customers'" 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 === 'customers' ? 'text-gray-800 dark:text-gray-100' : ''"
+                   href="/admin/customers">
+                    <span x-html="$icon('user-group')"></span>
+                    <span class="ml-4">Customers</span>
+                </a>
+            </li>
+
+            <!-- Marketplace -->
             <li class="relative px-6 py-3">
                 <span x-show="currentPage === 'marketplace'" class="absolute inset-y-0 left-0 w-1 bg-purple-600 rounded-tr-lg rounded-br-lg" aria-hidden="true"></span>
                 <a class="inline-flex items-center w-full text-sm font-semibold transition-colors duration-150 hover:text-gray-800 dark:hover:text-gray-200"
                    :class="currentPage === 'marketplace' ? 'text-gray-800 dark:text-gray-100' : ''"
                    href="/admin/marketplace">
-                    <span x-html="$icon('shopping-bag')"></span>
-                    <span class="ml-4">Marketplace Import</span>
+                    <span x-html="$icon('globe')"></span>
+                    <span class="ml-4">Marketplace</span>
                 </a>
             </li>
         </ul>