feat: add Clients page with order history

Groups orders by phone into a client list with expandable rows showing per-order history, summary cards for total clients and closed revenue, and a Clients nav item in the sidebar.

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

src/App.jsx

@@ -18,6 +18,7 @@ const AnalyticsDashboard = React.lazy(() => import('./admin/analytics/AnalyticsD
 const CompletedOrdersTable = React.lazy(() => import('./admin/analytics/CompletedOrdersTable'));
 const UserManagementPage = React.lazy(() => import('./admin/users/UserManagementPage'));
 const SettingsPage = React.lazy(() => import('./admin/settings/SettingsPage'));
+const ClientsPage = React.lazy(() => import('./admin/clients/ClientsPage'));
 
 function PublicSite() {
   return (
@@ -52,6 +53,7 @@ function App() {
               <Route path="completed" element={<CompletedOrdersTable />} />
               <Route path="users" element={<UserManagementPage />} />
               <Route path="settings" element={<SettingsPage />} />
+              <Route path="clients" element={<ClientsPage />} />
             </Route>
           </Route>
         </Routes>