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/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
-    "main.css": "/static/css/main.67d2dbc4.css",
-    "main.js": "/static/js/main.1e7808e0.js",
+    "main.css": "/static/css/main.d4111563.css",
+    "main.js": "/static/js/main.f56b4907.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
-    "main.67d2dbc4.css.map": "/static/css/main.67d2dbc4.css.map",
-    "main.1e7808e0.js.map": "/static/js/main.1e7808e0.js.map",
+    "main.d4111563.css.map": "/static/css/main.d4111563.css.map",
+    "main.f56b4907.js.map": "/static/js/main.f56b4907.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
-    "static/css/main.67d2dbc4.css",
-    "static/js/main.1e7808e0.js"
+    "static/css/main.d4111563.css",
+    "static/js/main.f56b4907.js"
   ]
 }