feat: Support Multi-Currency Accounting

[xiaobanni]

Prerequisites

This requirement was mentioned in #25 , but there has been no progress for a long time. I am emphasizing it here and suggesting some possible solutions.

Describe the Feature Request

Problem
The current app does not support multi-currency accounting. For example, if I purchase an item using USD and record the transaction as 5 USD, and later change the app’s currency in the settings to EUR, the item’s price is incorrectly displayed as 5 EUR. The original currency (USD) is not preserved, nor is the amount dynamically converted to the correct value (e.g., 4.78 EUR based on the exchange rate).

Proposed Solution
1. Each record should store the currency type (e.g., USD, EUR) in the database.
2. When the system currency is changed, the app should dynamically convert the prices of past records based on the current exchange rate or keep them displayed in their original currency.
3. Enable users to select the currency type when adding a new record, rather than relying solely on the global setting in the app.
4. Allow users to configure commonly used currency combinations in the settings for quicker access.

Describe the Use Case

This feature would have a wide audience, including users who travel abroad, shop internationally, or work overseas.

Additional Information

No response