feat(admin): Administration page — Users tab + Server tab (hardware, per-user CPU/memory/home share)

/users becomes /admin (redirected), a tab shell over the existing Users view
and a new Server tab backed by GET /api/host (server/lib/host-stats.js):
hostname/OS/uptime, CPU and load, memory and swap, every real disk with
HDD/SSD, NVIDIA GPUs, and each user's live share from two /proc samples —
attributed by the member's Linux uid (S3) or, for host processes, by the
inherited AGENTHUB_LANE_ID's workspace owner. Read-only, staff-only (/host
in STAFF_ONLY_PREFIXES), comm names only. Members' tmux card moves to the
Server tab.

client/src/components/Sidebar.tsx

@@ -73,7 +73,7 @@ import {
   ChevronDown,
   FolderTree,
   LogOut,
-  UsersRound,
+  ShieldCheck,
 } from "lucide-react";
 import type { LucideIcon } from "lucide-react";
 import { api } from "../lib/api";
@@ -479,11 +479,11 @@ export function Sidebar({
           {[
             // The file manager serves a shared server; locally it is the desktop's.
             ...NAV_KEYS.filter((n) => !(perms.local && n.to === "/files")),
-            // Server mode: the Users page, for an admin or analyst.
+            // Server mode: Administration (users, server), for an admin or analyst.
             ...(auth.state.mode === "server" &&
             auth.state.user &&
             auth.state.user.role !== "developer"
-              ? [{ to: "/users", icon: UsersRound, key: "nav:users" }]
+              ? [{ to: "/admin", icon: ShieldCheck, key: "nav:admin" }]
               : []),
           ].map(({ to, icon: Icon, key }) => {
             const label = t(key);