Improve Currency Conversion & Primary Currency logic

**Describe the usability problem**
In _Settings_ -> _General_ the description of Currency Conversion says that will display fiat values throughout the application, and actually could be crypto value as well. The Primary Currency, says "Select Fiat to prioritize displaying values in your selected fiat currency". When actually I can have selected one crypto value.

**Screenshots**
(https://recordit.co/S8gEegdgBE)

**To Reproduce**
1. Go to Settings
2. Click on General
3. Select Fiat in Primary Currency
4. Change currency conversion value to one crypto value (1ST for example)
5. Go to the wallet left menu option
6. Can see that no fiat values are shown, only crypto values

**Expected behaviour**
When we talk about Fiat the user will think about Fiat values and not crypto values.

**User impact | Usage frequency | Feasibility scores**
Small user impact, just a detail for UX

Start by reproducing the flow in Settings > General: select Fiat, choose a crypto value such as 1ST, then inspect the wallet view. No source file or test is named, so trace the settings and wallet display logic first. Done means the Fiat wording and behavior consistently match the values shown.

Written by the indexing model from the issue text.