Migrate MainActivity layout to a full Compose NavHost

[alexistercero-rho-dev]

• Convert MainActivity from AppCompatActivity to ComponentActivity and replace XML layouts with setContent.
• Remove activity_main.xml and nav_graph.xml, migrating navigation logic to a Compose-based NavHost.
• Implement routing using LaunchedEffect to reactively navigate between LoginScreen and HomeScreen based on authentication state.
• Replace fragment-based UI with LoginScreen and HomeScreen composables.
• Introduce LoadingScreen and LoadingOverlay composables to manage UI state during session initialization and background processing.
• Clean up View-based dependencies including Data Binding and NavHostFragment.