Add orderly shutdown on window close

src/core/messages.rs

@@ -11,6 +11,10 @@ pub enum CoreCommand {
     /// for a NEW room; it's ignored when joining (the label rides in the ticket).
     Join { name: String, ticket: String, room_name: String, input_device: Option<String>, output_device: Option<String>, echo_cancellation: bool, avatar: crate::avatar::Avatar },
     Leave,
+    /// Orderly app shutdown: finalize recordings, leave any active room, stop local
+    /// audio/screen-share work, close the persistent network stack, then ack with
+    /// [`UiEvent::ShutdownComplete`].
+    Shutdown,
     ToggleMute,
     /// Change our avatar (W4) and re-announce it to the room over presence.
     SetAvatar(crate::avatar::Avatar),
@@ -126,5 +130,7 @@ pub enum UiEvent {
     /// Discoverable. Distinct from a user-driven change so the GUI knows to update
     /// without having issued the command itself.
     PresenceModeReverted { mode: PresenceMode },
+    /// Core finished orderly app shutdown and the GUI can exit.
+    ShutdownComplete,
     Error(String),
 }