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.

server/routes/host.js

@@ -0,0 +1,23 @@
+/**
+ * @file host.js
+ * @description `GET /api/host` — the Administration page's Server tab: the
+ * machine's hardware, load and disks, and each AgentHub user's share of its
+ * CPU and memory right now (server/lib/host-stats.js). Read-only; staff only
+ * in server mode (`/host` is in STAFF_ONLY_PREFIXES, server/lib/auth.js).
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+const { Router } = require("express");
+const { hostStats } = require("../lib/host-stats");
+
+const router = Router();
+
+router.get("/", async (_req, res) => {
+  try {
+    res.json(await hostStats());
+  } catch (e) {
+    res.status(500).json({ error: { code: "EHOST", message: e.message } });
+  }
+});
+
+module.exports = router;