ui: update chip + per-host button

- Surface UpdateAvailable + TargetVersion on the dashboard host row,
  the host_chrome header, and the JSON Host shape.
- New host_update_chip partial renders an amber out-of-date pill
  next to the agent-version display when the host's agent trails
  the server.
- Host detail right-rail gains an admin-only Update agent button
  (disabled when host is offline or already updating).
- New .update-chip and .btn-amber CSS tokens; tailwind output
  refreshed.

web/templates/partials/host_row.html

@@ -14,7 +14,7 @@
     {{- end -}}
   </div>
   <div class="mono {{if eq $h.Status "offline"}}text-ink-mid{{else}}text-ink{{end}} font-medium">{{$h.Name}}</div>
-  <div class="mono text-ink-mid text-[12px]">{{$h.OS}}/{{$h.Arch}}</div>
+  <div class="mono text-ink-mid text-[12px]">{{$h.OS}}/{{$h.Arch}}{{if .UpdateAvailable}} {{template "host_update_chip" .}}{{end}}</div>
   <div class="text-xs text-ink-mid">
     {{- if $h.CurrentJobID -}}
       <span class="text-accent">backup running…</span><br>