feat(docker): swarm topology view + image/disk usage page (milestone 77 #942)

Two new read-only sub-pages, linked from the Docker index header only when the
data exists (non-swarm / Docker-less installs aren't offered empty pages):

- /plugins/docker/swarm — services with replica health (running/desired,
  colour-coded green/amber/red), Swarm nodes (role/availability/status, leader
  badge), and task→node placement with node ids resolved to hostnames. Grouped
  by reporting manager host. Empty state explains manager-only collection.

- /plugins/docker/disk — per-host reclaimable space, image/layer/container/
  volume/build-cache sizes, stopped-container count, and a per-image table
  (size, shared, ref count, reclaimable badge). Notes prune actions are deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/docker/templates/docker/index.html

@@ -2,7 +2,15 @@
 {% block title %}Docker — Steward{% endblock %}
 {% block content %}
 <div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1.5rem;gap:1rem;flex-wrap:wrap;">
-  <h1 class="page-title" style="margin-bottom:0;">Docker</h1>
+  <div style="display:flex;align-items:baseline;gap:1rem;">
+    <h1 class="page-title" style="margin-bottom:0;">Docker</h1>
+    {% if has_swarm %}
+    <a href="/plugins/docker/swarm" style="font-size:0.85rem;color:var(--accent);text-decoration:none;">Swarm →</a>
+    {% endif %}
+    {% if has_disk %}
+    <a href="/plugins/docker/disk" style="font-size:0.85rem;color:var(--accent);text-decoration:none;">Disk →</a>
+    {% endif %}
+  </div>
   {% include "_time_range.html" %}
 </div>