core (constants): fix XDG config path:

The config path was using `APP_NAME` instead of `APP_NAME_FILE`, resulting in `Libervia`
(with uppercase `L`) used instead of `libervia`.

libervia/backend/core/constants.py

@@ -59,7 +59,7 @@ class Const:
         Path("/etc/_"),
         Path("/etc/"),
         Path.home(),
-        xdg_config_home / APP_NAME,
+        xdg_config_home / APP_NAME_FILE,
         # We use backend root as last config path, this is useful for per instance
         # configurations.
         Path(backend.__file__).parent.parent,