Pause menu functionality (#89)

* make menu use actual soundsettings values
* allow changing volume in pause menu
* persist saving settings

crates/hedz_reloaded/src/client/settings.rs

@@ -0,0 +1,42 @@
+use bevy::prelude::*;
+use bevy_pkv::prelude::*;
+
+use crate::client::audio::SoundSettings;
+
+#[derive(Resource)]
+struct SaveTimer(Timer);
+
+pub fn plugin(app: &mut App) {
+    app.insert_resource(PkvStore::new("Rustunit", "HEDZ"));
+    app.insert_resource(SaveTimer(Timer::from_seconds(1.0, TimerMode::Once)));
+
+    app.add_systems(Update, persist_settings);
+    app.add_systems(Startup, load_settings);
+}
+
+fn persist_settings(
+    settings: Res<SoundSettings>,
+    mut pkv: ResMut<PkvStore>,
+    mut timer: ResMut<SaveTimer>,
+    time: Res<Time>,
+) -> Result {
+    if settings.is_changed() {
+        timer.0.reset();
+    }
+
+    timer.0.tick(time.delta());
+
+    if timer.0.just_finished() {
+        pkv.set("audio", &*settings)?;
+    }
+
+    Ok(())
+}
+
+fn load_settings(mut settings: ResMut<SoundSettings>, pkv: Res<PkvStore>) -> Result {
+    if let Ok(loaded) = pkv.get::<SoundSettings>("audio") {
+        *settings = loaded;
+    }
+
+    Ok(())
+}