Fix: Add ensure_ascii=False to WhatsApp send payload

The send() payload contains user message content (msg.content) which
may include non-ASCII characters (e.g. CJK, German umlauts, emoji).

The auth frame and Discord heartbeat/identify payloads are left
unchanged as they only carry ASCII protocol fields.

nanobot/channels/whatsapp.py

@@ -87,7 +87,7 @@ class WhatsAppChannel(BaseChannel):
                 "to": msg.chat_id,
                 "text": msg.content
             }
-            await self._ws.send(json.dumps(payload))
+            await self._ws.send(json.dumps(payload, ensure_ascii=False))
         except Exception as e:
             logger.error(f"Error sending WhatsApp message: {e}")