ComposeView: Add close button and improve state handling
Add an explicit "Close" button to the compose view sheet. This allows users to dismiss the sheet without sending a post.
When the sheet is closed, its state is now properly reset. This includes clearing any entered text, selected media, and reply/quote context. Additionally, the keyboard is hidden and focus is cleared when the sheet is hidden or partially expanded.
The Proguard configuration was also updated to no longer keep all of Ktor's classes, reducing the final application size.