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/browse.html

@@ -17,7 +17,6 @@
       {% endif %}
       <a href="/ansible/inventory/targets" class="btn btn-ghost btn-sm">Inventory</a>
       <a href="/ansible/schedules" class="btn btn-ghost btn-sm">Schedules</a>
-      <a href="/ansible/" class="btn btn-ghost btn-sm">← Run History</a>
     </div>
 </div>
 
@@ -35,7 +34,6 @@
                 <button type="submit" class="btn btn-sm btn-danger">Delete</button>
             </form>
             {% endif %}
-            <a href="/ansible/browse" class="btn btn-ghost btn-sm">← Back to browse</a>
         </div>
     </div>
     <pre style="background:var(--bg);padding:1rem;border-radius:4px;overflow-x:auto;font-size:0.8rem;color:var(--text-muted);max-height:600px;overflow-y:auto;">{{ view_contents }}</pre>