Sign a PSBT and Broadcast the TX from the UI

**Is your feature request related to a problem? Please describe.**

Some features are not available in Jam yet (for example fee rates below 1 sat/vByte). In these cases the transaction has to be prepared in another wallet, like Sparrow, instead. But then there is no easy way to sign it with the JoinMarket wallet from the Jam UI.

**Describe the solution you'd like**

An "Import/Sign PSBT" option in the UI: import a PSBT (paste it or upload a file), sign it with the wallet, and get back the signed transaction, with the option to copy it, download it, or broadcast it directly.

**Describe alternatives you've considered**

Signing via CLI on the node:

```
docker exec -it jam wallet-tool.py /root/.joinmarket/wallets/jam_wallet.jmdat signpsbt
```

This works, but these steps are not easy for many users to do.

**Additional context**

Related to #633, a similar request for another feature (message signing) that is only available via CLI, not from the UI.

Start by tracing the existing Jam UI wallet and transaction flows, then compare them with the documented wallet-tool.py signpsbt command. Done means users can paste or upload a PSBT, sign it with the wallet, copy or download the signed transaction, and optionally broadcast it from the UI.

Written by the indexing model from the issue text.