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/host_agent/templates/settings_list.html

@@ -5,7 +5,6 @@
 {% block content %}
 <div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1rem;gap:1rem;flex-wrap:wrap;">
   <h1 class="page-title" style="margin-bottom:0;">Agent fleet</h1>
-  <a href="/hosts/" class="btn btn-ghost btn-sm">← Hosts</a>
 </div>
 <p style="color:var(--text-muted);font-size:0.84rem;margin:-0.4rem 0 1.25rem;">
   Bulk agent operations across your inventory. For a single host, use its