ESP-NOW: STA interface, notify browser on send failure

- Activate STA interface before ESP-NOW to fix ESP_ERR_ESPNOW_IF
- Notify browser on send failure: WebSocket sends error JSON; preset API returns 503
- Use exceptions for failure (not return value) to avoid false errors when send succeeds
- presets.js: handle server error messages in WebSocket onmessage

Made-with: Cursor

src/static/presets.js

@@ -25,6 +25,18 @@ const getEspnowSocket = () => {
     espnowPendingMessages = [];
   };
 
+  espnowSocket.onmessage = (event) => {
+    try {
+      const data = JSON.parse(event.data);
+      if (data && data.error) {
+        console.error('ESP-NOW:', data.error);
+        alert('ESP-NOW send failed. ' + (data.error === 'ESP-NOW send failed' ? 'Check device WiFi/interface.' : data.error));
+      }
+    } catch (_) {
+      // Ignore non-JSON or non-error messages
+    }
+  };
+
   espnowSocket.onclose = () => {
     espnowSocketReady = false;
     espnowSocket = null;