feat: admin panel login, layout, and styles

Add the admin CRM panel foundation including:
- Login/logout authentication flow with CSRF protection
- Sidebar layout with navigation for all CRM modules
- Comprehensive admin.css (1680+ lines) with components for
  cards, tables, badges, forms, buttons, alerts, modals,
  pagination, search, responsive breakpoints, and print styles
- admin.js with sidebar toggle, confirm dialogs, auto-dismiss
  alerts, and table search filtering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin/includes/admin-footer.php

@@ -0,0 +1,7 @@
+        </div><!-- /.admin-content -->
+    </div><!-- /.admin-main -->
+
+    <div class="sidebar-overlay" id="sidebarOverlay"></div>
+    <script src="../assets/js/admin.js"></script>
+</body>
+</html>