Post ScreenResume to default screen.

When the default screen is first created it was not getting the event ScreenResume. All other screens receive a ScreenResume when first created and _all_ screens (the default one and custom screens) receive this event when they become the active screen again, so this was kind of an edge case that needed the event to be posted by hand.
Related comments: #2581

src/textual/app.py

@@ -2133,6 +2133,7 @@ async def on_event(self, event: events.Event) -> None:
             screen = Screen(id="_default")
             self._register(self, screen)
             self._screen_stack.append(screen)
+            screen.post_message(events.ScreenResume())
             await super().on_event(event)
 
         elif isinstance(event, events.InputEvent) and not event.is_forwarded: