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/Helper/MouseSensitivity.cs

@@ -2,40 +2,16 @@ using UnityEngine;
 
 /// Global mouse look-around sensitivity multiplier, shared by every camera
 /// controller (FreeCam, FirstPerson, ThirdPersonOrbit). 1 = unchanged baseline.
-/// Persisted per-machine via PlayerPrefs so the setting survives restarts.
+/// Persisted via GameSettings (Settings.json) so the setting survives restarts.
 public static class MouseSensitivity
 {
     public const float Default = 1f;
     public const float Min = 0.1f;
     public const float Max = 4f;
 
-    const string PrefKey = "Settings.MouseSensitivity";
-
-    static bool _loaded;
-    static float _multiplier = Default;
-
     public static float Multiplier
     {
-        get
-        {
-            if (!_loaded)
-            {
-                _loaded = true;
-                _multiplier = Mathf.Clamp(PlayerPrefs.GetFloat(PrefKey, Default), Min, Max);
-            }
-            return _multiplier;
-        }
-        set
-        {
-            float v = Mathf.Clamp(value, Min, Max);
-            _loaded = true;
-            if (Mathf.Approximately(v, _multiplier)) return;
-            _multiplier = v;
-            PlayerPrefs.SetFloat(PrefKey, v);
-            // Flush to disk now so the setting survives a hard kill (build), not
-            // only a clean quit. Settings changes are rare user actions, so the
-            // extra write is negligible.
-            PlayerPrefs.Save();
-        }
+        get => Mathf.Clamp(GameSettings.MouseSensitivity, Min, Max);
+        set => GameSettings.MouseSensitivity = Mathf.Clamp(value, Min, Max);
     }
 }