Draw front lines on the react map.

https://github.com/dcs-liberation/dcs_liberation/issues/2039

game/server/frontlines/models.py

@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+from game.server.leaflet import LeafletPoint
+
+
+class FrontLineJs(BaseModel):
+    extents: list[LeafletPoint]