Add gamma control and night light (#38)

Add configurable night light, smooth gamma transitions, and shell controls. Update documentation and panel tests, and fix portal test startup synchronization.

crates/ferese-config/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod activities;
 pub mod families;
 pub mod file;
+pub mod night_light;
 pub mod notifications;
 pub use ferese_panel::panel;
 pub mod presets;