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/_macros.html

@@ -0,0 +1,18 @@
+{# Shared template macros. Import with: {% from "_macros.html" import crumbs %} #}
+
+{# Breadcrumb trail. `items` is a list of (label, href) pairs, ordered root→current.
+   Intermediate items with a truthy href render as links; the last item (or any
+   item with an empty href) renders as plain current text. #}
+{% macro crumbs(items) -%}
+<nav class="breadcrumb" aria-label="Breadcrumb">
+  {%- for label, href in items -%}
+    {%- if loop.last -%}
+      <span class="breadcrumb-cur" aria-current="page">{{ label }}</span>
+    {%- elif href -%}
+      <a href="{{ href }}">{{ label }}</a><span class="breadcrumb-sep">›</span>
+    {%- else -%}
+      <span>{{ label }}</span><span class="breadcrumb-sep">›</span>
+    {%- endif -%}
+  {%- endfor -%}
+</nav>
+{%- endmacro %}