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- New host_update_chip partial renders an amber out-of-date pill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_chrome.html

@@ -83,7 +83,7 @@
       <div class="flex items-center gap-3 mt-3 text-[13px] text-ink-mute">
         <span class="mono text-ink-mid">{{$host.OS}}/{{$host.Arch}}</span>
         <span class="text-ink-fade">·</span>
-        <span>agent <span class="mono text-ink-mid">{{if $host.AgentVersion}}{{$host.AgentVersion}}{{else}}—{{end}}</span></span>
+        <span>agent <span class="mono text-ink-mid">{{if $host.AgentVersion}}{{$host.AgentVersion}}{{else}}—{{end}}</span>{{if $page.UpdateAvailable}} {{template "host_update_chip" $page}}{{end}}</span>
         <span class="text-ink-fade">·</span>
         <span>restic <span class="mono text-ink-mid">{{if $host.ResticVersion}}{{$host.ResticVersion}}{{else}}—{{end}}</span></span>
         <span class="text-ink-fade">·</span>