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/auth.ts

@@ -35,14 +35,19 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
           });
         }
 
-        return { id: String(user.id), name: user.username };
+        return { id: String(user.id), name: user.username, rank: user.rank };
       },
     }),
   ],
   callbacks: {
-    // NextAuth stores the user id in token.sub automatically; surface it on the session.
+    jwt({ token, user }) {
+      if (user) token.rank = (user as { rank?: number }).rank;
+      return token;
+    },
+    // NextAuth stores the user id in token.sub automatically; surface id + rank.
     session({ session, token }) {
       if (token.sub && session.user) session.user.id = token.sub;
+      if (typeof token.rank === "number" && session.user) session.user.rank = token.rank;
       return session;
     },
   },