feat(homecore-api): negotiate modern websocket clients

v2/crates/homecore-api/src/rest.rs

@@ -381,6 +381,7 @@ pub async fn compatibility(
             "states_services_config": "implemented",
             "events": "implemented",
             "render_template": "implemented",
+            "feature_negotiation_and_panels": "implemented",
             "registries": "requires_registry_backend",
             "lovelace_media": "integration_dependent"
         }

v2/crates/homecore-api/src/ws.rs

@@ -240,6 +240,12 @@ impl Connection {
 
     async fn handle_cmd(&self, cmd: WsCommand, tx: &tokio::sync::mpsc::Sender<String>) {
         match cmd.kind.as_str() {
+            "supported_features" => {
+                // HOMECORE currently emits individual messages. Accepting the
+                // negotiation command keeps modern HA clients compatible while
+                // deliberately declining optional coalescing.
+                self.ack(tx, cmd.id, true, None);
+            }
             "ping" => {
                 let msg = serde_json::json!({"id": cmd.id, "type": "pong"});
                 let _ = tx.try_send(msg.to_string());
@@ -258,6 +264,11 @@ impl Connection {
                 });
                 self.ack(tx, cmd.id, true, Some(payload));
             }
+            "get_panels" => {
+                // Panels are frontend integration resources. An empty map is
+                // the valid shape for a headless server.
+                self.ack(tx, cmd.id, true, Some(serde_json::json!({})));
+            }
             "get_services" => {
                 let services = self.state.homecore().services().registered_services().await;
                 let mut by_domain: std::collections::HashMap<