feat(health): System Health in three groups, a score that sees lag; admin user detail

Dashboard → System Health is grouped into AgentHub, Machine and Agent
activity. The machine (GET /api/host, every signed-in user) is drawn by
components/health/HostHealth.tsx: host card beside the CPU/memory history,
then CPU (threads as compact cells + temperatures/GPUs), disks & network,
busiest processes, and per-user tiles. Concurrency bars each show their own
agent type on hover.

The Health Score (client/src/lib/healthScore.ts) is now 100 minus what each
check costs, between an amber and a red threshold: event-loop p99
(server/lib/loop-delay.js, shared with /api/metrics), Linux PSI pressure for
CPU, memory and disk I/O (/api/host `pressure`; load per core without PSI),
RAM, the database's disk, V8 heap, hooks silent while agents work, and agent
errors in the last 24 h. All-time success rate, API errors and the transcript
cache hit rate no longer count. /api/settings/info adds server.event_loop and
activity {last_event_at, working_agents, agents_24h}.

Administration: a user row opens its detail in place (GET
/api/users/:id/details — spend, account pool share, live panes, workspaces,
newest sessions); the server panel is gone, member tmux folded into the table;
the Sessions list shows each row's owner in server mode.

server/lib/loop-delay.js

@@ -0,0 +1,36 @@
+/**
+ * @file loop-delay.js
+ * @description How long this server's event loop was held up — what an
+ * operator sees as a laggy dashboard or terminal: every request, hook and
+ * keystroke echo waits for it. One `monitorEventLoopDelay` histogram for the
+ * whole process, sampled every 20 ms and reset each minute, so `loopDelay()`
+ * describes the last full minute (the running one until the first minute
+ * ends). Read by `GET /api/metrics` (Prometheus gauges) and
+ * `GET /api/settings/info` (the System Health score).
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+const { monitorEventLoopDelay } = require("perf_hooks");
+
+const WINDOW_MS = 60_000;
+const histogram = monitorEventLoopDelay({ resolution: 20 });
+histogram.enable();
+
+const read = () => ({
+  p50: histogram.percentile(50),
+  p99: histogram.percentile(99),
+  max: histogram.max,
+});
+
+let lastWindow = null; // the last full minute, reported until the next one ends
+setInterval(() => {
+  lastWindow = read();
+  histogram.reset();
+}, WINDOW_MS).unref();
+
+/** `{p50, p99, max}` in nanoseconds over the last minute. */
+function loopDelay() {
+  return lastWindow || read();
+}
+
+module.exports = { loopDelay, WINDOW_MS };