Unify default pricing currency selection

[aerosol]

Changes

This PR queries Paddle prices with customer IP, in order to utilise their geolocation-based currency selection. Additionally, /api/paddle/currency endpoint is exposed, so that currency displayed on the website is the same.

Tests

• [x] Automated tests have been added
• [ ] This PR does not require tests

Changelog

• [ ] Entry has been added to changelog
• [x] This PR does not make a user-facing change

Documentation

• [ ] Docs have been updated
• [x] This change does not need a documentation update

Dark mode

• [ ] The UI has been tested both in dark and light mode
• [x] This PR does not change the UI