Add host connection status check endpoint and UI indicator

- Implement handle_connection_status API to test host port reachability
- Display Host:Up/Down status in the UI

src/static/webssh.css

@@ -134,6 +134,16 @@ body {
   color: #9ca3af;
 }
 
+.host-status {
+  margin-top: 4px;
+  font-size: 11px;
+  font-weight: 500;
+}
+
+.host-status.up { color: #34d399; }
+.host-status.down { color: #f87171; }
+.host-status.checking { color: #fbbf24; }
+
 .connection-status {
   display: inline-block;
   width: 8px;