Make invited room behavior prettier and sleeker

Although the room InviteScreen is fully functional and works properly, there are still some ways we could make it a bit nicer looking and sleeker.

• Upon showing the InviteScreen (in set_displayed_invite()), change the join button icon to an "enter room" icon, and set the button as enabled and the button text to be "Join room".
• Once the join button has been clicked, change the icon to a loading spinner and the text to "Joining...".
  • This will require a special button type that can show other non-icon content. Alternatively, we could hide the button temporarily and just show a LoadingSpinner with a background frame that looks like a disabled button... that might be easier.
• Once the JoinRoomResultAction::Joined has been received, change the button icon to a green checkmark: " ✅ Joined!"
• Close rooms when they are successfully Left.
  • We can now do this because navigation between rooms has recently been implemented. See how we achieve this to close a tombstoned room when its successor has been joined: https://github.com/project-robius/robrix/blob/d7117c3414f7472e8a5d618b7e063ec8f6d7f1f8/src/app.rs#L536-L593

Start with the InviteScreen flow at set_displayed_invite() and the handling of JoinRoomResultAction::Joined. Read src/app.rs around lines 536-593 for the existing room-closing behavior after navigation. Done means the invite button shows the requested enabled, loading, and joined states, and successfully left rooms close as specified.