webui: fixes for machines endpoint

- fix machines settings file location
- raise 404 if machine doesn't exist
- improve machine api test

pkgs/clan-cli/tests/test_machines_config.py

@@ -0,0 +1,8 @@
+from pathlib import Path
+
+from clan_cli.config import machine
+
+
+def test_schema_for_machine(machine_flake: Path) -> None:
+    schema = machine.schema_for_machine("machine1", machine_flake)
+    assert "properties" in schema