refactor: standardize admin templates with shared macros

Migrate all admin templates to use standardized components:
- Use tabs macros (tabs_nav, tab_button) for tab navigation
- Use number_stepper for quantity/numeric inputs
- Use headers macros for consistent page layouts
- Use modals macros for dialog components

Affected pages: dashboard, settings, logs, content-pages, companies,
vendors, users, imports, marketplace, code-quality, and more.

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

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

app/templates/admin/customers.html

@@ -1,22 +1,13 @@
 {# app/templates/admin/customers.html #}
 {% extends "admin/base.html" %}
+{% from 'shared/macros/headers.html' import page_header %}
 
 {% block title %}Customers{% endblock %}
 
 {% block alpine_data %}adminCustomers(){% endblock %}
 
 {% block content %}
-<!-- Page Header -->
-<div class="flex items-center justify-between my-6">
-    <div>
-        <h2 class="text-2xl font-semibold text-gray-700 dark:text-gray-200">
-            Customers
-        </h2>
-        <p class="text-sm text-gray-600 dark:text-gray-400 mt-1">
-            Manage platform customers
-        </p>
-    </div>
-</div>
+{{ page_header('Customers', subtitle='Manage platform customers') }}
 
 <!-- Empty State -->
 <div class="px-4 py-12 bg-white rounded-lg shadow-md dark:bg-gray-800">