Walkthrough: Instant & Smooth Chat

I have optimized the app's loading performance and transition smoothness across the main navigation and chat room flows.

Key Changes

Native Splash Screen Integration: The app now uses the Android Splash Screen API to handle the initial authentication check. The splash screen remains visible until the app knows whether to show the login screen or the main chat list.
Removed Loading Spinner: Eliminated the secondary manual loading circle that appeared after the splash screen, creating a seamless transition to the main content.

Eager ViewModel Loading: ChatRoomsViewModel now starts fetching the room list from the local database as soon as it's created, rather than waiting for the UI to be ready.
Zero-Flash Empty State: Optimized the initial state of the room list to avoid flashing "No rooms" while the database is being queried.

Reactive Chat ViewModel: Refactored ChatViewModel to be fully reactive. It now combines room info, user status, and local messages into a single state derived directly from the database flows.
Instant Message Display: Messages stored in the local database are now shown immediately upon entering a chat room. The "Loading..." spinner flash has been removed for rooms with existing history.
Background Sync: Network synchronization for new messages now happens entirely in the background, ensuring the UI remains responsive and smooth.

Verified that the app boots directly to the room list without a secondary loading screen.
Verified that chat rooms show local history immediately.
Verified that shared element transitions for room names and avatars remain fluid.