feat(prometheus): the Prometheus dashboard widget, target + template (#5691)

#5691 "Generic Prometheus dashboard widget: pick a target and a template",
step 3 of milestone 508 "Prometheus widget templates".

- /plugins/prometheus/widget draws one target with one template in widget
  shape: the verdict (led by "isn't answering" when the target is down),
  then each block with problems first, figures beside a share bar or the
  last 24h of bars, and the healthy rest folding to "+N more healthy" past
  six. A missing target or template, or one that no longer checks, says so.
- library.py: bundled templates from plugins/prometheus/widgets/*.yaml,
  the picker's target and template lists, and the default title.
- widgets/fabled_archives.yaml: the first bundled template, against the
  series Archives reports (refined in #5693).
- Dashboard: a widget setting may take `options_from` (read when the picker
  opens) with an `empty` sentence; `default_title`; a Details link may name
  settings ("/plugins/prometheus/targets/{target}"). Prometheus is grouped
  with the integrations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/dashboard/_widget_drawer.html

@@ -45,8 +45,12 @@
             {% for p in w.params %}
             <div>
               <label style="font-size:0.74rem;color:var(--text-muted);display:block;margin-bottom:0.2rem;">{{ p.label }}</label>
-              {% if p.type == "select" %}
-              <select name="param_{{ p.key }}" class="input">
+              {% if p.type == "select" and p.options_from and not p.options %}
+              {# A list read when the picker opens (a plugin's targets, its templates) that is empty. #}
+              <p class="field-hint" style="margin:0;">{{ p.empty or "Nothing to choose from yet." }}</p>
+              {% elif p.type == "select" %}
+              <select name="param_{{ p.key }}" class="input"{% if p.options_from %} required{% endif %}>
+                {% if p.options_from and p.options|length > 1 %}<option value="">— choose —</option>{% endif %}
                 {% for opt in p.options %}
                 <option value="{{ opt.value }}" {% if opt.value == p.default %}selected{% endif %}>{{ opt.label }}</option>
                 {% endfor %}