Move settings to Settings.json and add master volume

Replace the former per-value PlayerPrefs storage with a shared human-readable
Settings.json store (GameSettings). MouseSensitivity keeps its public .Multiplier
API and only swaps its backing to the JSON store, so the camera controllers stay
untouched. Add a MasterVolume facade that drives the AudioMixer's exposed
"MasterVolume" parameter (loaded from Resources, linear<->dB mapping), applied at
startup and on change, plus a MasterVolumeSlider binder. The default is the level
currently authored on the mixer instead of an arbitrary constant.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/Components/UI/MasterVolumeSlider.cs

@@ -0,0 +1,37 @@
+using TMPro;
+using UnityEngine;
+using UnityEngine.UI;
+
+/// Binds a UI Slider (and optional value label) to the global MasterVolume level.
+/// The slider's range is authored in the inspector (0 - 1); this only reads/writes
+/// the value so scene-side range edits are respected.
+public class MasterVolumeSlider : MonoBehaviour
+{
+    [SerializeField] private Slider slider;
+    [Tooltip("Optional TMP label showing the current volume as a percentage (e.g. \"80%\").")]
+    [SerializeField] private TMP_Text valueLabel;
+
+    void OnEnable()
+    {
+        if (!slider) return;
+        slider.SetValueWithoutNotify(MasterVolume.Volume);
+        UpdateLabel(MasterVolume.Volume);
+        slider.onValueChanged.AddListener(OnSliderChanged);
+    }
+
+    void OnDisable()
+    {
+        if (slider) slider.onValueChanged.RemoveListener(OnSliderChanged);
+    }
+
+    void OnSliderChanged(float value)
+    {
+        MasterVolume.Volume = value;
+        UpdateLabel(value);
+    }
+
+    void UpdateLabel(float value)
+    {
+        if (valueLabel) valueLabel.text = Mathf.RoundToInt(value * 100f) + "%";
+    }
+}