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- New host_update_chip partial renders an amber out-of-date pill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/pages/host_detail.html

@@ -78,6 +78,26 @@
         </p>
       </div>
 
+      {{if and $page.CanAdmin $page.UpdateAvailable}}
+      <div class="panel rounded-[7px] px-4 py-3.5">
+        <div class="text-[11px] text-ink-fade uppercase tracking-[0.1em] mb-2.5">Agent update</div>
+        <p class="text-[12px] text-ink-mute leading-[1.55] mb-3">
+          Agent at <span class="mono text-ink-mid">{{$host.AgentVersion}}</span> ·
+          server at <span class="mono text-ink-mid">{{$page.TargetVersion}}</span>.
+          Pushes a self-update command; the agent re-launches into the new binary
+          and reconnects.
+        </p>
+        <form hx-post="/hosts/{{$host.ID}}/update" hx-swap="none">
+          <button class="btn btn-amber btn-block"
+                  {{if not $page.Online}}disabled title="Agent must be online"
+                  {{else if $page.UpdateInProgress}}disabled title="Update already in progress"
+                  {{end}}>
+            Update agent
+          </button>
+        </form>
+      </div>
+      {{end}}
+
       <div class="panel rounded-[7px] px-4 py-3.5">
         <div class="text-[11px] text-ink-fade uppercase tracking-[0.1em] mb-2.5">Restore</div>
         <p class="text-[12px] text-ink-mute leading-[1.55] mb-3">