testnet4 support

does specter desktop support `testnet4` ?

I get in this case addresses with prefix `ert1` which bitcoin core cannot deal with:

```
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: [2026-01-03 22:05:45,816] ERROR in specter_error: Request error for method getreceivedbyaddress('ert1q7eppjgvgfupx5vhycatpglrh709cgkn6p3y3uq', 0): Invalid Bitcoin address
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: Traceback (most recent call last):
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: File "/usr/lib/python3.12/site-packages/cryptoadvance/specter/wallet/wallet.py", line 764, in getdata
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: value_on_address = self.rpc.getreceivedbyaddress(self.change_address, 0)
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: File "/usr/lib/python3.12/site-packages/cryptoadvance/specter/rpc.py", line 509, in fn
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: raise RpcError(
Jan 03 22:05:45 raspberrypi5-c8-bf-64 specter[601]: cryptoadvance.specter.rpc.RpcError: Request error for method getreceivedbyaddress('ert1q7eppjgvgfupx5vhycatpglrh709cgkn6p3y3uq', 0): Invalid Bitcoin address

```