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/share_view.html

@@ -5,6 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>{{ dashboard.name }} — Fabled Scryer</title>
   <script src="https://unpkg.com/htmx.org@2.0.4/dist/htmx.min.js"></script>
+  <script src="https://cdn.jsdelivr.net/npm/chart.js@4.4.4/dist/chart.umd.min.js"></script>
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link href="https://fonts.googleapis.com/css2?family=Libertinus+Serif:ital,wght@0,400;0,600;0,700;1,400&display=swap" rel="stylesheet">
@@ -75,11 +76,11 @@
     {% 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>
       </div>
-      {# Pass share token as ?s= param so require_role allows through #}
+      {# Pass share token as &s= param so require_role allows through #}
       <div id="widget-{{ item.db.id }}"
-           hx-get="{{ item.defn.hx_url }}?s={{ raw_token }}"
+           hx-get="{{ item.hx_url }}&s={{ raw_token }}"
            hx-trigger="load{% if item.defn.poll %}, every {{ poll_interval }}s{% endif %}"
            hx-swap="innerHTML">
       </div>