Rework App Launch

🎯 The objective

Decouple the App Launch from the Message List and establish a standalone MainActivity that manages app state and navigation.

🦮 Rules of the Road

🟢 In Scope

• Decoupling of MainActivity from the Message List logic.
• Creation of a new MainActivity as the primary entry point and single point for navigation decisions.
• Integration of FeatureLauncherActivity functionality into the new MainActivity.
• Implementation of a unified navigation strategy using a Navigation Library. (Navigation 3?)
• Migration of app-level state management to a dedicated MainScreenViewModel.
• Handling of all external intents (Launcher, Notifications, Widgets) in the new MainActivity.
• Renaming the existing MainActivity to MessageListActivity.

🔴 Out of Scope

• Redesign of the Message List UI (beyond decoupling from the Activity).
• Migration of all app features to Compose (only the navigation shell is initially targeted).
• Changes to the backend or synchronization logic.

🚧 Technical and/or Business Requirements

Technical Requirements

• Use MVI architecture for the MainScreenViewModel.
• Ensure compatibility with existing Fragment-based destinations.
• The new MainActivity must be the designated LAUNCHER activity.

Business Requirements

• Maintain existing deep-linking and notification entry point functionality.
• Provide a more stable and maintainable foundation for future improvements.