Add admin foundation + Users resource (Filament replacement, slice 1)

Plain App Router admin (aligned to habbo-next, no Refine):
- rank surfaced on the NextAuth session; staff guard isStaff() [pure,
  unit-tested] + requireStaff() reading min_staff_rank, gating /admin.
- /admin dashboard (counts), /admin/users (paginated + search),
  /admin/users/[id] detail.
- src/actions/admin-users.ts: staff-gated server actions wiring the user editor
  to the existing services — giveCurrency (RCON or DB fallback), setMotto/setRank
  (DB + RCON), alertUser, disconnectUser.

Verified: tsc exit 0, vitest 45/45, next build exit 0 (/admin routes).

src/app/admin/page.tsx

@@ -0,0 +1,33 @@
+import { prisma } from "@/lib/prisma";
+
+export const dynamic = "force-dynamic";
+
+export default async function AdminDashboard() {
+  const [users, online, articles] = await Promise.all([
+    prisma.user.count(),
+    prisma.user.count({ where: { online: "1" } }),
+    prisma.websiteArticles.count(),
+  ]);
+
+  const stats = [
+    { label: "Users", value: users },
+    { label: "Online now", value: online },
+    { label: "Articles", value: articles },
+  ];
+
+  return (
+    <main>
+      <h1>Dashboard</h1>
+      <div className="grid cols-2">
+        {stats.map((s) => (
+          <div key={s.label} className="card">
+            <h2 style={{ margin: 0 }}>{s.value}</h2>
+            <p className="muted" style={{ margin: 0 }}>
+              {s.label}
+            </p>
+          </div>
+        ))}
+      </div>
+    </main>
+  );
+}