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

plugins/snmp/templates/snmp/device.html

@@ -1,9 +1,10 @@
 {# plugins/snmp/templates/snmp/device.html #}
 {% extends "base.html" %}
+{% from "_macros.html" import crumbs %}
 {% block title %}SNMP — {{ device_name }} — Steward{% endblock %}
+{% block breadcrumb %}{{ crumbs([("SNMP", "/plugins/snmp/"), (device_name, "")]) }}{% endblock %}
 {% block content %}
 <div style="display:flex;align-items:center;gap:1rem;margin-bottom:1.5rem;flex-wrap:wrap;">
-  <a href="/plugins/snmp/" style="color:var(--text-muted);font-size:0.85rem;">&#8592; SNMP</a>
   <h1 class="page-title" style="margin:0;">{{ device_name }}</h1>
   <span style="font-size:0.8rem;color:var(--text-dim);">{{ device.host }}</span>
   <div style="margin-left:auto;display:flex;gap:0.5rem;">