fix: lock action calls moonlock directly instead of loginctl

loginctl lock-session requires a D-Bus listener that is
difficult to set up reliably. Direct moonlock invocation
is simpler and works immediately.
Also removes CSS fade-in animation (low-fps on layer shell).

tests/test_power.py

@@ -13,11 +13,11 @@ class TestLock:
     """Tests for the lock power action."""
 
     @patch("moonset.power.subprocess.run")
-    def test_calls_loginctl_lock_session(self, mock_run) -> None:
+    def test_calls_moonlock(self, mock_run) -> None:
         lock()
 
         mock_run.assert_called_once_with(
-            ["loginctl", "lock-session"], check=True, timeout=POWER_TIMEOUT
+            ["moonlock"], check=True, timeout=POWER_TIMEOUT
         )
 
     @patch("moonset.power.subprocess.run")