Base currencies for Portfolio screen

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.

Start by locating the Portfolio screen and its current input and summary-function entry points. Review how currency selection and dialogs are handled before defining the [add_base] control, the base-currency editing dialog, and hints for each input. Done means users can choose one to three base currencies, see equivalent hints, and retain the summary function's current behavior.

Written by the indexing model from the issue text.