isShowingAttachments

Gives us information if we're showing the attachments picker or not.

Backed by both MutableState (for Compose observation/recomposition) and SavedStateHandle (for persistence across process death). This ensures the picker remains visible after activity recreation (e.g. when returning from an external activity like the camera or file picker under "Don't keep activities" mode), which in turn allows rememberLauncherForActivityResult callbacks inside the picker composable tree to re-register and receive the pending result.