Timeline: Use ViewModel state for refresh status
The `isRefreshing` state in `MainView` is now derived directly from the `TimelineViewModel`'s `isFetchingMoreTimeline` and `isFetchingMoreNotifications` properties. This removes the local `mutableStateOf` and simplifies state management.
Additionally, timeline and notification data is now fetched on ViewModel initialization instead of via a `LaunchedEffect` in the view.