Surface screen share failures instead of silently doing nothing

toggleScreenSharing only had `.catch(logger.error)`, so a getDisplayMedia
request that hangs (element-call-rageshakes#17152: Element Desktop on
Windows, the user pressed the screen share button 14 times in 25 seconds
and the log shows nothing but the toggle lines and livekit-client's
"waiting for pending publication promise timed out") left the user with
a button that does nothing and us with no evidence of why.

Log when a toggle is requested and when it completes or fails, with the
elapsed time, so a hang is visible in the logs. Explicit failures other
than the user cancelling the picker show as a non-modal "Could not start
screen sharing" toast. Nothing is inferred from a toggle taking a long
time: the user may simply be choosing what to share.

src/room/InCallView.tsx

@@ -270,6 +270,7 @@ export const InCallView: FC<InCallViewProps> = ({
   const audioParticipants = useBehavior(vm.livekitRoomItems$);
   const participantCount = useBehavior(vm.participantCount$);
   const reconnecting = useBehavior(vm.reconnecting$);
+  const screenShareError = useBehavior(vm.screenShareError$);
   const layout = useBehavior(vm.layout$);
   const edgeToEdge = useBehavior(vm.edgeToEdge$);
   const overflowing = useBehavior(vm.overflowing$);
@@ -399,6 +400,21 @@ export const InCallView: FC<InCallViewProps> = ({
       );
   }
 
+  const onDismissScreenShareToast = useCallback(
+    () => vm.dismissScreenShareError(),
+    [vm],
+  );
+  const screenShareToast = (
+    <Toast
+      onDismiss={onDismissScreenShareToast}
+      open={screenShareError !== null}
+      autoDismiss={5000}
+      modal={false}
+    >
+      {t("error.screen_share_failed")}
+    </Toast>
+  );
+
   // The reconnecting toast cannot be dismissed
   const onDismissReconnectingToast = useCallback(() => {}, []);
   // We need to use a non-modal toast to avoid trapping focus within the toast.
@@ -634,6 +650,7 @@ export const InCallView: FC<InCallViewProps> = ({
       <ReactionsAudioRenderer vm={vm} muted={muteAllAudio} />
       <RingingAudioRenderer vm={ringingVm} muted={muteAllAudio} />
       {reconnectingToast}
+      {screenShareToast}
       {earpieceOverlay}
       <ReactionsOverlay vm={vm} />
       {footer}