fix(ui): unify header/breadcrumb treatment, drop redundant back buttons

The page-top chrome was inconsistent: most nested views used the breadcrumb
kicker + page-title pattern, but plugin sub-pages used ad-hoc "← back" links,
~11 pages stacked a breadcrumb AND a redundant ancestor back button, and two
(monitors/edit, hosts/uptime) had a back button but no breadcrumb. The
settings/plugin_detail page stacked all of it at once.

Unify on the breadcrumb-led model:
- settings/_tabs.html: drop the hardcoded "Settings" h1; the breadcrumb
  ("Settings › …") plus the tab strip is the header.
- settings/plugin_detail: drop the "← Plugins" back button.
- docker container_detail/swarm/disk + snmp/device: replace ad-hoc back links
  with the standard crumbs() breadcrumb.
- host_agent, ansible/*, alerts/maintenance: remove redundant ancestor back
  buttons (the breadcrumb's parent crumbs already link there); keep lateral
  shortcuts (Inventory/Schedules/Browse/Targets/Groups/New).
- monitors/edit, hosts/uptime: add the missing breadcrumb, drop the back link.

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

steward/templates/ansible/run_detail.html

@@ -4,7 +4,6 @@
 {% block breadcrumb %}{{ crumbs([("Ansible", "/ansible/"), ("Run " ~ run.id[:8], "")]) }}{% endblock %}
 {% block content %}
 <div style="display:flex;align-items:center;gap:1rem;margin-bottom:1.5rem;">
-    <a href="/ansible/" class="btn btn-ghost btn-sm">← Runs</a>
     <h1 class="page-title" style="margin-bottom:0;">Run Detail</h1>
     <div style="margin-left:auto;display:flex;gap:0.5rem;">
         {% if run.status.value in ("running", "queued") and session.user_role in ("operator", "admin") %}