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/layout.tsx

@@ -0,0 +1,24 @@
+import Link from "next/link";
+import type { ReactNode } from "react";
+import { requireStaff } from "@/lib/admin/guard";
+
+export const dynamic = "force-dynamic";
+
+export default async function AdminLayout({ children }: { children: ReactNode }) {
+  const staff = await requireStaff();
+
+  return (
+    <div style={{ display: "grid", gridTemplateColumns: "190px 1fr", gap: "1.5rem" }}>
+      <aside className="card" style={{ height: "fit-content" }}>
+        <p className="muted" style={{ marginTop: 0 }}>
+          Staff · {staff.username} (rank {staff.rank})
+        </p>
+        <nav style={{ display: "grid", gap: "0.45rem" }}>
+          <Link href="/admin">Dashboard</Link>
+          <Link href="/admin/users">Users</Link>
+        </nav>
+      </aside>
+      <section>{children}</section>
+    </div>
+  );
+}