Feature Request - Support for "Historical Cost" Exchange Rates and Automatic FX Gain/Loss Tracking

I often exchange CNY for foreign currencies in advance. I want my bookkeeping to reflect my actual exchange cost instead of the market's real-time rate.

Proposed Workflow:

1. Exchange Record: Allow users to log a manual currency exchange (e.g., CNY to USD at a specific rate).

2. Expense Logging: When logging a foreign currency expense, the app should use that fixed historical rate.

3. Automatic Split: The system should automatically split the transaction into:

• The Expense: The foreign amount converted to the default currency at the real-time rate.

• FX Gain/Loss: A separate entry representing the difference between the real-time rate and my historical fixed rate.