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/night_light.rs

@@ -0,0 +1,26 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Deserialize, Serialize)]
+#[serde(default, deny_unknown_fields)]
+pub struct Settings {
+    pub enabled: bool,
+    pub temperature: u16,
+}
+
+impl Default for Settings {
+    fn default() -> Self {
+        Self {
+            enabled: false,
+            temperature: 4500,
+        }
+    }
+}
+
+impl Settings {
+    pub fn validate(self) -> Result<Self, String> {
+        if !(1000..=6500).contains(&self.temperature) {
+            return Err("night-light temperature must be between 1000 and 6500 K".into());
+        }
+        Ok(self)
+    }
+}