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>

steward/templates/base.html

@@ -145,6 +145,13 @@ textarea { resize: vertical; }
 /* Empty state */
 .empty { color: var(--text-muted); font-size: 0.9rem; padding: 2rem; text-align: center; }
 
+/* Breadcrumbs */
+.breadcrumb { display:flex; align-items:center; flex-wrap:wrap; gap:0.35rem; font-size:0.8rem; color:var(--text-dim); margin-bottom:1rem; }
+.breadcrumb a { color:var(--text-muted); }
+.breadcrumb a:hover { color:var(--text); }
+.breadcrumb-sep { color:var(--border-mid); }
+.breadcrumb-cur { color:var(--text-muted); }
+
 /* Stat cards (dashboard) */
 .stat-val { font-size: 2rem; font-weight: 700; line-height: 1; }
 .stat-label { font-size: 0.8rem; color: var(--text-muted); margin-left: 0.3rem; }
@@ -228,6 +235,7 @@ function setTimeRange(val) {
 </script>
 
     <main>
+        {% block breadcrumb %}{% endblock %}
         {% if plugin_failures and session.user_role == 'admin' %}
         <div style="background:color-mix(in srgb,var(--yellow) 12%,var(--bg-elevated));
                     border:1px solid color-mix(in srgb,var(--yellow) 35%,var(--border));