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/lib/admin/is-staff.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+import { isStaff } from "./is-staff";
+
+describe("isStaff", () => {
+  it("is true at or above the min staff rank", () => {
+    expect(isStaff(7, 7)).toBe(true);
+    expect(isStaff(10, 7)).toBe(true);
+  });
+
+  it("is false below the min staff rank", () => {
+    expect(isStaff(6, 7)).toBe(false);
+    expect(isStaff(1, 7)).toBe(false);
+  });
+});