core/greeter: embed base config, tie enable into sync, sync config

argument always, preserve existing args

core/internal/config/embedded/niri-greeter.kdl

@@ -0,0 +1,17 @@
+hotkey-overlay {
+    skip-at-startup
+}
+
+environment {
+    DMS_RUN_GREETER "1"
+}
+
+gestures {
+    hot-corners {
+        off
+    }
+}
+
+layout {
+    background-color "#000000"
+}