Add total user balances stat card to admin dashboard

src/app/admin/dashboard/page.tsx

@@ -16,6 +16,7 @@ export default async function AdminDashboard() {
     homeworkPending,
     homeworkTotal,
     progressTotal,
+    balanceAggregate,
   ] = await Promise.all([
     prisma.user.count({ where: { role: "student" } }),
     prisma.user.count({ where: { role: "student", createdAt: { gte: monthAgo } } }),
@@ -30,8 +31,11 @@ export default async function AdminDashboard() {
     prisma.homeworkSubmission.count({ where: { feedbacks: { none: {} } } }),
     prisma.homeworkSubmission.count(),
     prisma.lessonProgress.count(),
+    prisma.balanceTransaction.aggregate({ _sum: { amount: true } }),
   ]);
 
+  const totalBalance = Number(balanceAggregate._sum.amount ?? 0);
+
   // Recent enrollments
   const recentEnrollments = await prisma.courseEnrollment.findMany({
     orderBy: { enrolledAt: "desc" },
@@ -58,7 +62,7 @@ export default async function AdminDashboard() {
       </p>
 
       {/* Stats grid */}
-      <div className="grid grid-cols-2 md:grid-cols-4 gap-4 mb-8">
+      <div className="grid grid-cols-2 lg:grid-cols-5 gap-4 mb-8">
         <StatCard
           label="Студентов"
           value={totalStudents}
@@ -85,6 +89,12 @@ export default async function AdminDashboard() {
           subAccent={homeworkPending > 0}
           href="/curator/homework"
         />
+        <StatCard
+          label="На балансах"
+          value={totalBalance.toLocaleString("ru-RU", { minimumFractionDigits: 2, maximumFractionDigits: 2 }) + " ₽"}
+          sub="сумма по всем пользователям"
+          href="/admin/users"
+        />
       </div>
 
       <div className="grid md:grid-cols-2 gap-6">
@@ -179,7 +189,7 @@ function StatCard({
   href,
 }: {
   label: string;
-  value: number;
+  value: number | string;
   sub?: string;
   subAccent?: boolean;
   href?: string;