Add returning customers stat to admin users page

Introduces a new 'Returning Customers' card to the admin users dashboard, showing the count and percentage of users with more than one order. Also updates the grid layout to accommodate the new card and imports the necessary icon.

components/admin/AdminAnalytics.tsx

@@ -12,7 +12,6 @@ import { fetchClient } from "@/lib/api-client";
 import { BarChart as RechartsBarChart, Bar, XAxis, YAxis, CartesianGrid, Tooltip, ResponsiveContainer, LineChart, Line, ComposedChart } from 'recharts';
 import { formatGBP } from "@/utils/format";
 
-// API response data structure
 interface AnalyticsData {
   vendors?: {
     total?: number;