feat(ui): breadcrumb navigation across nested pages

Adds a breadcrumb trail to nested views for orientation. Mechanism: a
{% block breadcrumb %} slot in base.html (+ styling) and a shared crumbs()
macro in templates/_macros.html; each nested page fills the block with its
trail (root→current, last item is the current page). Pages without the block
render no bar, so top-level nav roots stay clean.

Applied to: Ansible (browse, schedules, playbook editor, run detail) +
inventory (targets/groups + detail), host_agent (fleet, host detail,
settings), hosts form, settings tabs (ansible/auth/notifications/plugins/
reports + plugin detail), dashboard (list, edit), and alerts (rule form,
maintenance + new). Dynamic labels (host/target/run/dashboard names) come
from the page context — no route changes. All 60 templates Jinja-compile.

Task #873.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/host_agent/templates/host_detail.html

@@ -1,5 +1,7 @@
 {% extends "base.html" %}
+{% from "_macros.html" import crumbs %}
 {% block title %}{{ host.name }} — Host Agent — Steward{% endblock %}
+{% block breadcrumb %}{{ crumbs([("Host Agents", "/plugins/host_agent/"), (host.name, "")]) }}{% endblock %}
 
 {% macro fmt_bps(v) %}
 {%- if v is none -%}—

plugins/host_agent/templates/host_list.html

@@ -1,5 +1,7 @@
 {% extends "base.html" %}
+{% from "_macros.html" import crumbs %}
 {% block title %}Host Agents — Steward{% endblock %}
+{% block breadcrumb %}{{ crumbs([("Dashboard", "/"), ("Host Agents", "")]) }}{% endblock %}
 {% block content %}
 <div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1.25rem;gap:1rem;flex-wrap:wrap;">
   <h1 class="page-title" style="margin-bottom:0;">Host Agents</h1>

plugins/host_agent/templates/settings_list.html

@@ -1,5 +1,7 @@
 {% extends "base.html" %}
+{% from "_macros.html" import crumbs %}
 {% block title %}Host Agent — Steward{% endblock %}
+{% block breadcrumb %}{{ crumbs([("Host Agents", "/plugins/host_agent/"), ("Settings", "")]) }}{% endblock %}
 {% block content %}
 <h1 class="page-title">Host Agent — Registered Hosts</h1>