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/monitors/edit.html

@@ -1,10 +1,11 @@
 {% extends "base.html" %}
+{% from "_macros.html" import crumbs %}
 {% from "monitors/_fields.html" import type_fields, toggle_script %}
 {% block title %}Edit Monitor — Steward{% endblock %}
+{% block breadcrumb %}{{ crumbs([("Monitors", "/monitors/"), ("Edit monitor", "")]) }}{% endblock %}
 {% block content %}
 <div style="margin-bottom:1.25rem;">
-  <a href="/monitors/" style="color:var(--text-muted);font-size:0.85rem;">← Monitors</a>
-  <h1 class="page-title" style="margin:0.4rem 0 0;">Edit Monitor</h1>
+  <h1 class="page-title" style="margin-bottom:0;">Edit Monitor</h1>
 </div>
 
 {% set inp = "width:100%;padding:0.4rem 0.65rem;background:var(--bg);border:1px solid var(--border-mid);border-radius:4px;color:var(--text);font-size:0.88rem;" %}