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/disk.html

@@ -0,0 +1,84 @@
+{# docker/disk.html — image/disk usage: reclaimable space, per-image sizes #}
+{% extends "base.html" %}
+{% block title %}Disk — Docker — Steward{% endblock %}
+{% block content %}
+
+<div style="margin-bottom:1rem;">
+  <a href="/plugins/docker/" style="font-size:0.82rem;color:var(--text-muted);text-decoration:none;">← All containers</a>
+</div>
+<h1 class="page-title" style="margin-bottom:0.4rem;">Image &amp; disk usage</h1>
+<p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.5rem;">
+  Reclaimable = space held by images no container references. Cleanup actions
+  (prune) arrive in a later release — these are read-only figures for now.
+</p>
+
+{% if host_groups %}
+{% for g in host_groups %}
+<div style="margin-bottom:2rem;">
+  <div style="font-weight:600;font-size:0.95rem;margin-bottom:0.75rem;">{{ g.host_name }}</div>
+
+  {# ── Summary stats ────────────────────────────────────────────────────── #}
+  <div style="display:grid;grid-template-columns:repeat(auto-fill,minmax(150px,1fr));gap:0.75rem;margin-bottom:1rem;">
+    <div class="card" style="margin-bottom:0;">
+      <div class="section-title" style="margin-bottom:0.3rem;">Reclaimable</div>
+      <span class="stat-val" style="color:{% if g.summary.images_reclaimable != '0 B' %}var(--orange){% else %}var(--green){% endif %};">{{ g.summary.images_reclaimable }}</span>
+    </div>
+    <div class="card" style="margin-bottom:0;">
+      <div class="section-title" style="margin-bottom:0.3rem;">Images</div>
+      <span class="stat-val">{{ g.summary.images_size }}</span>
+      <div style="font-size:0.74rem;color:var(--text-muted);">{{ g.summary.images_active }}/{{ g.summary.images_total }} in use</div>
+    </div>
+    <div class="card" style="margin-bottom:0;">
+      <div class="section-title" style="margin-bottom:0.3rem;">Stopped</div>
+      <span class="stat-val" style="{% if g.stopped %}color:var(--text-muted){% endif %};">{{ g.stopped }}</span>
+    </div>
+    <div class="card" style="margin-bottom:0;">
+      <div class="section-title" style="margin-bottom:0.3rem;">Writable layers</div>
+      <span class="stat-val">{{ g.summary.containers_size }}</span>
+      <div style="font-size:0.74rem;color:var(--text-muted);">{{ g.summary.containers_count }} containers</div>
+    </div>
+    <div class="card" style="margin-bottom:0;">
+      <div class="section-title" style="margin-bottom:0.3rem;">Volumes</div>
+      <span class="stat-val">{{ g.summary.volumes_size }}</span>
+      <div style="font-size:0.74rem;color:var(--text-muted);">{{ g.summary.volumes_count }} volumes</div>
+    </div>
+    <div class="card" style="margin-bottom:0;">
+      <div class="section-title" style="margin-bottom:0.3rem;">Build cache</div>
+      <span class="stat-val">{{ g.summary.build_cache_size }}</span>
+    </div>
+  </div>
+
+  {# ── Per-image table ──────────────────────────────────────────────────── #}
+  <div class="card-flush">
+    <table class="table">
+      <thead>
+        <tr><th>Image</th><th>Size</th><th>Shared</th><th>Containers</th></tr>
+      </thead>
+      <tbody>
+        {% for im in g.images %}
+        <tr>
+          <td style="font-size:0.84rem;font-family:ui-monospace,monospace;max-width:340px;white-space:nowrap;overflow:hidden;text-overflow:ellipsis;">
+            {{ im.repo_tag }}
+            {% if im.reclaimable %}<span title="not referenced by any container" style="font-size:0.68rem;color:var(--orange);border:1px solid var(--orange);border-radius:3px;padding:0 0.3rem;margin-left:0.4rem;">reclaimable</span>{% endif %}
+          </td>
+          <td style="font-family:ui-monospace,monospace;font-size:0.86rem;">{{ im.size }}</td>
+          <td style="font-family:ui-monospace,monospace;font-size:0.86rem;color:var(--text-muted);">{{ im.shared }}</td>
+          <td style="font-size:0.86rem;color:{% if im.containers %}var(--text){% else %}var(--text-muted){% endif %};">{{ im.containers }}</td>
+        </tr>
+        {% else %}
+        <tr><td colspan="4" style="color:var(--text-muted);font-size:0.85rem;text-align:center;padding:1.5rem;">No images reported.</td></tr>
+        {% endfor %}
+      </tbody>
+    </table>
+  </div>
+</div>
+{% endfor %}
+{% else %}
+<div class="card" style="text-align:center;padding:3rem;">
+  <div style="color:var(--text-muted);font-size:0.9rem;">
+    No disk usage reported yet. The host agent reports image/disk usage from
+    <code>docker system df</code> on hosts running Docker.
+  </div>
+</div>
+{% endif %}
+{% endblock %}