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/main.py

@@ -99,7 +99,13 @@ async def main(port=80):
                     print("WS received raw:", data)
 
                 # Forward raw JSON payload over ESPNow to configured peers
-                await esp.send(data)
+                try:
+                    await esp.send(data)
+                except Exception:
+                    try:
+                        await ws.send(json.dumps({"error": "ESP-NOW send failed"}))
+                    except Exception:
+                        pass
             else:
                 break