Send fails during sync

```
Traceback (most recent call last):
File "/usr/local/bin/coldcore", line 3268, in
main()
File "/usr/local/bin/coldcore", line 3259, in main
cli.run()
File "/usr/local/bin/coldcore", line 318, in run
return fnc(*func_args, **func_kwargs)
File "/usr/local/bin/coldcore", line 2163, in prepare_send
return _prepare_send(config, rpcw, to_address, amount, spend_from_list)
File "/usr/local/bin/coldcore", line 2837, in _prepare_send
result = rpcw.walletcreatefundedpsbt(
File "/usr/local/bin/coldcore", line 625, in _call_wrapper
return self._call(name, *args)
File "/usr/local/bin/coldcore", line 575, in _call
raise JSONRPCError(
__main__.JSONRPCError: msg: 'Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable -fallbackfee.' code: -4
```

Start at the coldcore CLI path through prepare_send and _prepare_send, then inspect the walletcreatefundedpsbt call and the reported JSONRPCError. Reproduce the failure with fallbackfee disabled and establish the expected behavior for send during sync; done means the reported failure is handled or no longer occurs under the relevant conditions.

Written by the indexing model from the issue text.