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/index.js

@@ -129,6 +129,7 @@ const authRouter = require("./routes/auth");
 const meRouter = require("./routes/me");
 const accountsRouter = require("./routes/accounts");
 const usersRouter = require("./routes/users");
+const hostRouter = require("./routes/host");
 const { auditGuard } = require("./lib/audit");
 const { authGuard, isServerMode, hasAdmin } = require("./lib/auth");
 const { scopeGuard } = require("./lib/scope");
@@ -179,6 +180,7 @@ function createApp() {
   app.use("/api/me", meRouter);
   app.use("/api/accounts", accountsRouter);
   app.use("/api/users", usersRouter);
+  app.use("/api/host", hostRouter);
 
   app.use("/api/sessions", sessionsRouter);
   app.use("/api/agents", agentsRouter);