feat(dashboard): merge top summary strip + Status widget into one Overview

Operator chose a single canonical summary. Repurpose the status_overview
widget into 'Overview' (host count + monitor up/down/pending + Alerts link;
key kept so existing dashboards upgrade in place) and remove the redundant
fixed top strip from the dashboard view. Drops _get_summary_stats and its
now-unused PingResult/DnsResult imports (rule 22).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/core/widgets.py

@@ -24,8 +24,8 @@ WIDGET_REGISTRY: dict[str, dict] = {
     },
     "status_overview": {
         "key": "status_overview",
-        "label": "Status — Overview",
-        "description": "Unified up/down/pending summary across ping, DNS, and HTTP monitors",
+        "label": "Overview",
+        "description": "At-a-glance summary — host count, monitor up/down/pending, and alerts",
         "hx_url": "/status/widget",
         "detail_url": "/status/",
         "plugin": None,