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`.
