actions [nfc]: Move operation functions into ZulipAction abstract class

Move high level operations like logOutAccount and markNarrowAsRead into
the new ZulipAction abstract class, per discussion in:
https://chat.zulip.org/#narrow/channel/516-mobile-dev-help/topic/.23F1317.20showErrorDialog/near/2080576

This makes it clearer at call sites that these methods combine API operations with UI feedback.

Author: chimnayajith

lib/widgets/action_sheet.dart

@@ -706,7 +706,7 @@ class MarkAsUnreadButton extends MessageActionSheetMenuItemButton {
 
   @override void onPressed() async {
     final narrow = findMessageListPage().narrow;
-    unawaited(markNarrowAsUnreadFromMessage(pageContext, message, narrow));
+    unawaited(ZulipAction.markNarrowAsUnreadFromMessage(pageContext, message, narrow));
   }
 }