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/routes/metrics.js

@@ -29,21 +29,8 @@ const { calculateCost, dailyTokenRows } = require("./pricing");
 
 const router = Router();
 
-// How long the event loop was held up — what an operator sees as a laggy
-// terminal: every keystroke's echo waits for it. Sampled every 20 ms and reset
-// each minute, so the gauges describe the last minute or so.
-const LOOP_WINDOW_MS = 60_000;
-const loopDelay = require("perf_hooks").monitorEventLoopDelay({ resolution: 20 });
-loopDelay.enable();
-let loopWindow = null; // the last full minute, reported until the next one ends
-setInterval(() => {
-  loopWindow = {
-    p50: loopDelay.percentile(50),
-    p99: loopDelay.percentile(99),
-    max: loopDelay.max,
-  };
-  loopDelay.reset();
-}, LOOP_WINDOW_MS).unref();
+// Event-loop delay over the last minute (server/lib/loop-delay.js).
+const { loopDelay } = require("../lib/loop-delay");
 
 // Resolved once at load — the dashboard version, surfaced as a build-info label
 // (the canonical Prometheus idiom for exposing a version string as a metric).
@@ -116,11 +103,7 @@ router.get("/", (_req, res) => {
     [{ value: process.memoryUsage().rss }]
   );
 
-  const loop = loopWindow || {
-    p50: loopDelay.percentile(50),
-    p99: loopDelay.percentile(99),
-    max: loopDelay.max,
-  };
+  const loop = loopDelay();
   appendMetric(
     out,
     "agenthub_event_loop_delay_seconds",