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/lib/auth.js

@@ -89,6 +89,8 @@ const STAFF_ONLY_PREFIXES = [
   "/alerts/rules",
   "/users",
   "/accounts",
+  // The machine's hardware and every user's CPU/memory share.
+  "/host",
 ];
 // The run form's `/` autocomplete lists commands and skills.
 const MEMBER_READ_ALLOW = ["/cc-config/commands", "/cc-config/skills"];