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/controllers/preset.py

@@ -179,14 +179,20 @@ async def send_presets(request, session):
             batch = test_batch
             last_msg = test_msg
         else:
-            await send_chunk(batch)
+            try:
+                await send_chunk(batch)
+            except Exception:
+                return json.dumps({"error": "ESP-NOW send failed"}), 503, {'Content-Type': 'application/json'}
             await asyncio.sleep_ms(SEND_DELAY_MS)
             messages_sent += 1
             batch = {name: preset_obj}
             last_msg = build_message(presets=batch, save=save_flag, default=default_id)
 
     if batch:
-        await send_chunk(batch)
+        try:
+            await send_chunk(batch)
+        except Exception:
+            return json.dumps({"error": "ESP-NOW send failed"}), 503, {'Content-Type': 'application/json'}
         await asyncio.sleep_ms(SEND_DELAY_MS)
         messages_sent += 1