authentication through OKAPI

_disclaimer: I'm using this issue as a testground for different LLMs, agents, etc. Be warned_

### Overview

The app has to let the user to sign in with OKAPI.

### About OKAPI and its APIs

OKAPI uses OAuth 1.0a for authentication.

Read [the introduction to OKAPI](https://opencaching.pl/okapi/introduction.html), especially the [Authentication Levels](https://opencaching.pl/okapi/introduction.html#auth_levels) paragraph.

there are the following endpoints in `services/oauth` "category":
- [access_token](https://opencaching.pl/okapi/services/oauth/access_token.html)
- [authorize](https://opencaching.pl/okapi/services/oauth/authorize.html)
- [request_token](https://opencaching.pl/okapi/services/oauth/request_token.html)

### Implementation

- Try to reduce the amount of custom code written. Less = better.
- Prefer cross-platform libraries. This is a Kotlin Multiplatform project, targeting iOS and Android.

Start by reading OKAPI's introduction, especially Authentication Levels, then review the services/oauth request_token, authorize, and access_token endpoints. Define the cross-platform authentication flow so users can sign in on both iOS and Android with minimal custom code.

Written by the indexing model from the issue text.