feat(dashboard): a curated default layout, merged duplicate widgets, a labelled Overview (#5213)

- New installs start from widgets.RECOMMENDED, packed row by row for the
  enabled plugins; the edit drawer can reset a dashboard to it.
- Host Agent — Resources folds into Hosts — Overview (which gains load 1m),
  UniFi — Devices into UniFi — Overview; migration 0032 moves existing
  dashboards onto the survivors.
- The Overview labels what it counts (hosts, monitors up/down/pending,
  alerts firing) and lists firing alerts and down monitors with since-times.
- Uptime polls every 5 minutes (poll_seconds) instead of every cycle.

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

steward/templates/dashboard/_grid_item.html

@@ -20,7 +20,7 @@
     </div>
     <div class="dash-cell-body" id="widget-{{ item.db.id }}"
          hx-get="{{ item.hx_url }}"
-         hx-trigger="load, retry{% if item.defn.poll %}, every {{ poll_interval }}s{% endif %}"
+         hx-trigger="load, retry{% if item.defn.poll %}, every {{ item.defn.get('poll_seconds') or poll_interval }}s{% endif %}"
          hx-request='{"timeout": 20000}'
          data-poll="{{ '1' if item.defn.poll else '0' }}"
          hx-swap="innerHTML"><div class="cell-state">Loading…</div></div>