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/users/MemberTmuxCard.tsx

@@ -1,6 +1,6 @@
 /**
  * @file MemberTmuxCard.tsx
- * @description The Users page's tmux card (one Linux user per account only,
+ * @description The Administration page's Server-tab tmux card (one Linux user per account only,
  * `GET /api/users/tmux`): each member's own tmux server — the version it runs
  * beside the installed one, its sessions, whether an agent of theirs is
  * working — so an admin sees who still runs an old tmux after an upgrade.