feat(ui): plugin get_nav() hook → sidebar Infrastructure links (slice 2)

Plugin UIs had no nav home (only dashboard widgets / typed URLs). Add an
optional get_nav() plugin export and surface it in the sidebar.

- plugin_manager: _PLUGIN_NAV registry + get_plugin_nav() getter + a tolerant
  _collect_plugin_nav() (missing hook = fine; raising/malformed = logged &
  skipped; idempotent per plugin so hot-reload re-runs cleanly). Collected in
  both the startup load path and the hot-reload path.
- app.py: inject plugin_nav into the template context, filtered to enabled
  plugins so a hot-disabled plugin's link can't linger before restart.
- base.html: render plugin links under the Infrastructure group, with the same
  request.path active-state treatment as core links.
- docker/snmp/unifi/traefik: each exports get_nav() → its /plugins/<name>/ view.
- Tests: collector behavior (collect, missing hook, reload-replace, malformed
  item, raising hook, sorted output).

With docker enabled, "Docker" now appears under Infrastructure → its fleet view.

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

steward/templates/base.html

@@ -287,7 +287,9 @@ body.dash-editing .widget-drawer { transform:translateY(0); }
       <div class="nav-group">
         <div class="nav-group-label">Infrastructure</div>
         <a href="/hosts/" {% if _p.startswith('/hosts') %}class="active" aria-current="page"{% endif %}>Hosts</a>
-        {# plugin links land here in slice 2 (get_nav hook) #}
+        {% for item in plugin_nav %}
+        <a href="{{ item.href }}" {% if _p.startswith(item.href.rstrip('/')) %}class="active" aria-current="page"{% endif %}>{{ item.label }}</a>
+        {% endfor %}
       </div>
       <div class="nav-group">
         <div class="nav-group-label">Monitoring</div>