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/GameSettings.cs

@@ -0,0 +1,83 @@
+using System;
+using System.IO;
+using UnityEngine;
+
+/// Shared JSON-backed store for the player-facing game settings, written human-readable
+/// to <persistentDataPath>/Settings.json. Replaces the former per-value PlayerPrefs storage.
+/// Loaded lazily on first access and cached in memory (reads are plain field access, so the
+/// per-frame Multiplier reads from the camera controllers never touch the disk). Each change is
+/// written immediately (settings edits are rare user actions), so a hard-killed build keeps them.
+///
+/// The typed domain facades (MouseSensitivity, MasterVolume) own the ranges/clamping and read/write
+/// through here; this class stays domain-agnostic and only persists the raw values.
+public static class GameSettings
+{
+    private const string FileName = "Settings.json";
+    private const int FileVersion = 1;
+
+    [Serializable]
+    private class Data
+    {
+        public int version = FileVersion;
+        public float mouseSensitivity = global::MouseSensitivity.Default;
+        public float masterVolume = global::MasterVolume.Default;
+    }
+
+    private static Data _data;
+    private static Data Store => _data ??= Load();
+
+    private static string SavePath() => Path.Combine(Application.persistentDataPath, FileName);
+
+    public static float MouseSensitivity
+    {
+        get => Store.mouseSensitivity;
+        set
+        {
+            if (Mathf.Approximately(Store.mouseSensitivity, value)) return;
+            Store.mouseSensitivity = value;
+            Save();
+        }
+    }
+
+    public static float MasterVolume
+    {
+        get => Store.masterVolume;
+        set
+        {
+            if (Mathf.Approximately(Store.masterVolume, value)) return;
+            Store.masterVolume = value;
+            Save();
+        }
+    }
+
+    private static Data Load()
+    {
+        string path = SavePath();
+        if (File.Exists(path))
+        {
+            try
+            {
+                var data = JsonUtility.FromJson<Data>(File.ReadAllText(path));
+                if (data != null) return data;
+            }
+            catch (Exception e)
+            {
+                Debug.LogWarning($"[GameSettings] Loading {FileName} failed, using defaults: {e.Message}");
+            }
+        }
+        return new Data();
+    }
+
+    private static void Save()
+    {
+        try
+        {
+            Directory.CreateDirectory(Application.persistentDataPath);
+            File.WriteAllText(SavePath(), JsonUtility.ToJson(Store, true));
+        }
+        catch (Exception e)
+        {
+            Debug.LogWarning($"[GameSettings] Saving {FileName} failed: {e.Message}");
+        }
+    }
+}