Fix bugs, UX rough edges, add ntfy.sh + network discovery

Bugs fixed:
- Status page: autoRefresh checkbox now actually pauses the interval
- Dashboard edit modal: hide URL field for heartbeat resources (was always shown as required)
- Dashboard: apply items_per_page, default_sort, and refresh_interval from settings on mount
- Dashboard: call loadResources() immediately on mount (was waiting 15s for fallback)
- Observability: listen for WS 'metrics' messages so tab updates live, not just on poll

UX improvements:
- Dashboard: replaced window.confirm() with inline Confirm/Cancel buttons for single and bulk delete
- Dashboard: skeleton rows shown while initial data loads (no more flash of empty state)
- Notifications: WebSocket moved from window global to useRef; exponential backoff on reconnect
- Notifications: replaced window.confirm() with inline Confirm/Cancel for Clear All
- ResourceDetail: initial API calls run in parallel via Promise.all

New features:
- ntfy.sh push notification channel: enable in Settings, enter topic, get mobile push alerts
- Network Discovery: Actions → Discover Network scans LAN for reachable hosts, probes HTTP ports, and lets you add any found host to monitoring in one click

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

client/src/Status.js

@@ -12,9 +12,10 @@ function Status() {
 
   useEffect(() => {
     loadStatusData();
-    const interval = setInterval(loadStatusData, 15000); // Refresh every 15 seconds for critical status
+    if (!autoRefresh) return;
+    const interval = setInterval(loadStatusData, 15000);
     return () => clearInterval(interval);
-  }, []);
+  }, [autoRefresh]);
 
   const loadStatusData = async () => {
     try {