Crypto and stablecoin currencies for accounts and transactions

I'd like to keep accounts whose balance is a stablecoin or coin (e.g. a USDT card, a BTC wallet) and log everyday spending from them as normal categorised transactions.

Securities/portfolios cover crypto as holdings, but not spending accounts: portfolio cash is limited to ISO currencies and its transactions carry no category and stay out of expenses and cash flow. Today a coffee paid with a USDT card has to be either several investment records or a USD transaction that loses the real amount and that day's rate.

What it would take:
- custom currencies per user (name, symbol, decimals; codes like USDT, BTC, SAT)
- per-currency precision (BTC 8 decimals, JPY 0, KWD 3, USDT 6)
- dated exchange rates from CoinGecko, a fixed peg (USDT = 1 USD) or manual entry

A working implementation exists in PR #757, #758 and #759 (parked for now).