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/users/[id]/page.tsx

@@ -0,0 +1,82 @@
+import Link from "next/link";
+import { notFound } from "next/navigation";
+import {
+  alertUser,
+  disconnectUser,
+  giveCurrency,
+  setMotto,
+  setRank,
+} from "@/actions/admin-users";
+import { prisma } from "@/lib/prisma";
+
+export const dynamic = "force-dynamic";
+
+export default async function AdminUserDetail({
+  params,
+}: {
+  params: Promise<{ id: string }>;
+}) {
+  const { id } = await params;
+  const userId = Number(id);
+  const user = await prisma.user.findUnique({
+    where: { id: userId },
+    select: { id: true, username: true, motto: true, rank: true, credits: true, look: true, online: true },
+  });
+  if (!user) notFound();
+
+  return (
+    <main>
+      <p className="muted">
+        <Link href="/admin/users">← Users</Link>
+      </p>
+      <h1>{user.username}</h1>
+      <p className="muted">
+        ID {user.id} · rank {user.rank} · {user.credits} credits ·{" "}
+        {user.online === "1" ? "Online" : "Offline"}
+      </p>
+
+      <div className="grid cols-2">
+        <form action={giveCurrency} className="card">
+          <input type="hidden" name="userId" value={user.id} />
+          <h3 style={{ marginTop: 0 }}>Give currency</h3>
+          <select name="type">
+            <option value="credits">Credits</option>
+            <option value="duckets">Duckets</option>
+            <option value="diamonds">Diamonds</option>
+            <option value="points">Points</option>
+          </select>{" "}
+          <input name="amount" type="number" min={1} placeholder="Amount" />{" "}
+          <button type="submit">Give</button>
+        </form>
+
+        <form action={setMotto} className="card">
+          <input type="hidden" name="userId" value={user.id} />
+          <h3 style={{ marginTop: 0 }}>Set motto</h3>
+          <input name="motto" defaultValue={user.motto} maxLength={127} />{" "}
+          <button type="submit">Save</button>
+        </form>
+
+        <form action={setRank} className="card">
+          <input type="hidden" name="userId" value={user.id} />
+          <h3 style={{ marginTop: 0 }}>Set rank</h3>
+          <input name="rank" type="number" min={1} defaultValue={user.rank} />{" "}
+          <button type="submit">Set</button>
+        </form>
+
+        <form action={alertUser} className="card">
+          <input type="hidden" name="userId" value={user.id} />
+          <h3 style={{ marginTop: 0 }}>Alert</h3>
+          <input name="message" placeholder="Message" />{" "}
+          <button type="submit">Send</button>
+        </form>
+
+        <form action={disconnectUser} className="card">
+          <input type="hidden" name="userId" value={user.id} />
+          <input type="hidden" name="username" value={user.username} />
+          <h3 style={{ marginTop: 0 }}>Disconnect</h3>
+          <button type="submit">Disconnect from hotel</button>
+        </form>
+      </div>
+    </main>
+  );
+}