Fix adding and removing waypoints in the UI.

This moves more of the logic out of the UI so mypy can actually spot
these mistakes more often.

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/2495.

game/ato/flight.py

@@ -91,6 +91,12 @@ class Flight(SidcDescribable):
     def flight_plan(self) -> FlightPlan[Any]:
         return self._flight_plan_builder.get_or_build()
 
+    def degrade_to_custom_flight_plan(self) -> None:
+        from .flightplans.custom import Builder as CustomBuilder
+
+        self._flight_plan_builder = CustomBuilder(self, self.flight_plan.waypoints[1:])
+        self.recreate_flight_plan()
+
     def __getstate__(self) -> dict[str, Any]:
         state = self.__dict__.copy()
         # Avoid persisting the flight state since that's not (currently) used outside