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/settings/_tabs.html

@@ -1,5 +1,6 @@
-{# settings/_tabs.html — include at top of each settings section #}
-<h1 class="page-title">Settings</h1>
+{# settings/_tabs.html — include at top of each settings section.
+   No section <h1>: the breadcrumb kicker ("Settings › …") names the section and
+   the tab strip is the visual header, so we don't stack a redundant title. #}
 <div style="display:flex;gap:0;border-bottom:1px solid var(--border-mid);margin-bottom:1.5rem;">
   {% set tabs = [
     ("general",       "General",        "/settings/general/"),

steward/templates/settings/plugin_detail.html

@@ -8,7 +8,6 @@
 {% include "settings/_tabs.html" %}
 
 <div style="display:flex;align-items:center;gap:1rem;margin-bottom:1.5rem;">
-  <a href="/settings/plugins/" class="btn btn-ghost btn-sm">← Plugins</a>
   <h1 class="page-title" style="margin-bottom:0;">{{ plugin.get('name', plugin._dir) }}</h1>
   <span style="color:var(--text-muted);font-size:0.82rem;">v{{ plugin.get('version', '?') }}</span>
   {% if fail_reason %}