feat(web): support scheduler push messages and enrich welcome screen

- Expand welcome screen from 3 to 6 example cards covering core capabilities
- Enable background polling on page load so scheduler task notifications are received in real-time
- Fix duplicate poll loops via generation-based cancellation, reduce poll frequency to 5s/10s
- Ensure equal card height and adjust layout position for better visual balance

channel/web/static/css/console.css

@@ -728,3 +728,17 @@
 .cfg-tip:hover::after {
     opacity: 1;
 }
+
+/* Example cards: equal height via flex stretch + fixed 2-line description area */
+.example-card {
+    display: flex;
+    flex-direction: column;
+}
+.example-card > p {
+    flex: 1;
+    display: -webkit-box;
+    -webkit-line-clamp: 2;
+    -webkit-box-orient: vertical;
+    overflow: hidden;
+    min-height: 2.5em;  /* ~2 lines at text-sm leading-relaxed */
+}