feat: drag-and-drop dashboard editing, alert dynamic fields

- Replace up/down arrow reorder buttons on dashboard edit page with
  SortableJS drag handles; add POST /d/<id>/edit/reorder endpoint
  accepting ordered widget ID array
- Add {% block extra_scripts %} hook to base.html for page-specific JS
- Add dynamic alert rule field loading via HTMX partial (_rule_fields)
  so metric/threshold fields update when source is changed
- Add docs/plugins/index.yaml.example showing catalog index format

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fabledscryer/templates/dashboard/index.html

@@ -66,11 +66,11 @@
   {% for item in widgets %}
   <div class="card ping-card" style="break-inside:avoid;margin-bottom:1rem;">
     <div class="widget-header">
-      <span class="widget-title">{{ item.defn.label }}</span>
+      <span class="widget-title">{{ item.title }}</span>
       <a href="{{ item.defn.detail_url }}" class="widget-link">Details →</a>
     </div>
     <div id="widget-{{ item.db.id }}"
-         hx-get="{{ item.defn.hx_url }}"
+         hx-get="{{ item.hx_url }}"
          hx-trigger="load{% if item.defn.poll %}, every {{ poll_interval }}s{% endif %}"
          hx-swap="innerHTML">
     </div>