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/lib/api.ts

@@ -569,6 +569,46 @@ export interface MemberTmux {
   users: MemberTmuxRow[];
 }
 
+/** One account's (or the host's / other accounts') live share of the machine (`GET /api/host`). */
+export interface HostUsageGroup {
+  /** `user:<id>`, `host` (the host account outside any workspace) or `system`. */
+  key: string;
+  kind: "user" | "host" | "system";
+  userId?: number;
+  username?: string;
+  /** % of the whole machine's CPU (100 = every core busy) over the sample. */
+  cpu: number;
+  /** Resident memory, bytes (shared pages count in every process). */
+  memory: number;
+  memoryPercent: number;
+  processes: number;
+  /** Busiest processes, by the kernel's short name only. */
+  top: { pid: number; name: string; cpu: number; memory: number }[];
+}
+/** `GET /api/host` — hardware, load, disks and per-user usage. */
+export interface HostStats {
+  at: string;
+  hostname: string;
+  os: { platform: string; release: string; distro: string | null; arch: string };
+  uptime: number;
+  cpu: { model: string | null; cores: number; load: number[] };
+  memory: { total: number; available: number; used: number; swapTotal: number; swapUsed: number };
+  disks: {
+    device: string;
+    mount: string;
+    type: string;
+    kind: "HDD" | "SSD" | null;
+    total: number;
+    used: number;
+    free: number;
+  }[];
+  /** The mount holding the users' homes, or null. */
+  homesMount: string | null;
+  gpus: { name: string; utilization: number; memoryUsed: number; memoryTotal: number }[];
+  /** null off Linux. */
+  usage: { cpu: number; sampleMs: number; groups: HostUsageGroup[] } | null;
+}
+
 /** One user's home folder on disk (`GET /api/users/storage`); null = not measured. */
 export type UserStorage = {
   bytes: number;
@@ -918,6 +958,10 @@ export const api = {
     /** DELETE /api/me/avatar — back to the initials. */
     removeAvatar: () => request<MyAccount>("/me/avatar", { method: "DELETE" }),
   },
+  /** GET /api/host — the Server tab: hardware and each user's CPU/memory share (staff). */
+  host: {
+    stats: () => request<HostStats>("/host"),
+  },
   // ───────────────────────── Server mode staff views (routes/users.js) ─────────────────────────
   users: {
     /** URL of one account's avatar (admin/analyst), versioned by `avatarAt` (null = none). */