Refactored UI components and extracted screens into separate files.

The UI logic from `MainActivity.kt` has been moved into a new `AppContent` composable, and individual screens (`LoginScreen`, `DashboardScreen`, `AccountSelectionScreen`) have been extracted to their own files within the `ui.screens` package.

Additional changes:
- Updated `MainActivity` to use the new `AppContent` structure and handled back navigation for new account logins.
- Fixed a field name mismatch in `RefreshRequest` within `APIService.kt`, changing `refreshToken` to `refresh_token`.
- Minor formatting update in `README.md`.

README.md

@@ -13,6 +13,8 @@ The app is fully open source, lightweight, and can run on small devices like Ras
 
 You can either build the app from source or download the apk _(will be available soon)_
 
+---
+
 ## License
 
 This work is marked <a href="https://creativecommons.org/publicdomain/zero/1.0/">CC0 1.0</a>