Base currencies for Portfolio screen

[kirillt]

In the Portfolio screen, designed for multi-input calculation, we should introduce the concept of base currencies. These are one to three user-chosen currencies, deemed as familiar or native. For example, it could be USD, RUB and KZT. The intent is that each screen input should carry a hint, illustrating the equivalent amount in the base currencies.

To achieve this, we propose:

1. An additional [add_base] button.
2. Helpful hints adjacent to each input.
3. A dialog enabling the modification of base currencies set. A limit of three currencies seems reasonable.

The summary function should maintain its current operation.