fleet-manager: host uptime metric + reboot-pending status

The manager lets an admin schedule a reboot (desired.reboot_at) but never showed
whether it had happened. Add host uptime to the health snapshot and derive a
reboot status from it.

- proto: HealthSnapshot.uptime_seconds (15); regenerated pb.go (manager + agent copy).
- agent: probe /proc/uptime -> uptime_seconds (with parser + test).
- manager: Health.UptimeSeconds cached in inventory; snapshotToHealth maps it;
  influx writes + whitelists uptime_seconds as a queryable field.
- UI: fmt.js dur() + rebootStatus() — boot_time = last_seen - uptime (manager clock
  frame, no host/manager sync needed), compared to reboot_at. Monitor host detail
  shows an "uptime" row and a "reboot" row: pending in X / overdue by X / rebooted.

Observability-only — works before the reboot ACTUATOR lands (deferred). Once a host
boots after the scheduled time (BMC/manual now, agent later), the badge flips to
"rebooted" automatically.

serving/fleet-manager/ui/src/pages/Monitor.svelte

@@ -3,7 +3,7 @@
   import Panel from '../components/Panel.svelte'
   import StatePill from '../components/StatePill.svelte'
   import Chart from '../components/Chart.svelte'
-  import { relTime, pct, num, healthState, shortSerial } from '../lib/fmt.js'
+  import { relTime, pct, num, healthState, shortSerial, dur, rebootStatus } from '../lib/fmt.js'
 
   let hosts = $state([])
   let error = $state('')
@@ -105,6 +105,11 @@
           <dt class="text-faint">selinux</dt><dd class="text-muted">{selected.health?.selinux || '—'}</dd>
           <dt class="text-faint">chrony</dt><dd class="text-muted">{selected.health?.chrony || '—'}</dd>
           <dt class="text-faint">kiosk</dt><dd class="text-muted">{selected.health?.kiosk_active ? 'active' : 'idle'} · {selected.usage?.session_uptime || '—'}</dd>
+          <dt class="text-faint">uptime</dt><dd class="text-muted">{selected.health?.uptime_seconds ? dur(selected.health.uptime_seconds) : '—'}</dd>
+          {#if rebootStatus(selected)}
+            {@const rb = rebootStatus(selected)}
+            <dt class="text-faint">reboot</dt><dd class={rb.state === 'overdue' ? 'text-amber-hot' : rb.state === 'pending' ? 'text-amber' : 'text-teal'}>{rb.label}</dd>
+          {/if}
           <dt class="text-faint">last seen</dt><dd class="text-muted">{selected.online ? 'live' : relTime(selected.last_seen)}</dd>
         </dl>
         <div class="mt-4 space-y-3">