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/MasterVolume.cs

@@ -0,0 +1,82 @@
+using UnityEngine;
+using UnityEngine.Audio;
+
+/// Global master audio volume (0 = mute .. 1 = full), applied to the "MasterVolume" exposed
+/// parameter of the AudioMixer (loaded from Resources). Persisted via GameSettings (Settings.json).
+/// Applied once at startup via a runtime-init hook so the saved level takes effect before the
+/// first scene's audio plays, and again on every change.
+public static class MasterVolume
+{
+    public const float Min = 0f;
+    public const float Max = 1f;
+
+    /// The default is the level currently authored on the mixer (its exposed-parameter value
+    /// converted back to linear), so a fresh install starts at the mixer's own setting instead of
+    /// an arbitrary constant. Falls back to FallbackDefault if the mixer/param is unavailable.
+    public static float Default => TryReadMixerLinear(out float linear) ? linear : .5f;
+
+    // Must match the exposed parameter name authored on the AudioMixer asset.
+    private const string ExposedParam = "MasterVolume";
+    // AudioMixer.mixer lives under a Resources/ folder -> load by name without extension.
+    private const string MixerResourcePath = "AudioMixer";
+    // The exposed param is the master group's attenuation in dB; below this the log curve dives
+    // to -inf, so -80 dB is the practical silence floor (matches the mixer's own lower bound).
+    private const float SilenceDb = -80f;
+
+    private static AudioMixer _mixer;
+    private static AudioMixer Mixer => _mixer ? _mixer : _mixer = Resources.Load<AudioMixer>(MixerResourcePath);
+
+    public static float Volume
+    {
+        get => Mathf.Clamp(GameSettings.MasterVolume, Min, Max);
+        set
+        {
+            float v = Mathf.Clamp(value, Min, Max);
+            GameSettings.MasterVolume = v;
+            Apply(v); // set unconditionally: keeps the live mixer in sync even if the stored value was unchanged
+        }
+    }
+
+    // A perceptual volume slider is linear 0..1; the mixer wants dB. 20*log10 maps 1 -> 0 dB and
+    // fades smoothly, with 0 pinned to the silence floor (log10(0) is -inf).
+    private static float LinearToDb(float linear) =>
+        linear <= Min ? SilenceDb : Mathf.Max(SilenceDb, Mathf.Log10(linear) * 20f);
+
+    // Inverse of LinearToDb: read the mixer's authored dB back into the slider's 0..1 range.
+    private static float DbToLinear(float db) =>
+        db <= SilenceDb ? Min : Mathf.Clamp(Mathf.Pow(10f, db / 20f), Min, Max);
+
+    // Reads the mixer's current exposed-parameter level (from the active snapshot) as linear 0..1.
+    // Returns false if the mixer isn't loaded or the parameter isn't exposed, so callers can fall back.
+    private static bool TryReadMixerLinear(out float linear)
+    {
+        var mixer = Mixer;
+        if (!mixer || !mixer.GetFloat(ExposedParam, out float db))
+        {
+            linear = .5f;
+            return false;
+        }
+        linear = DbToLinear(db);
+        return true;
+    }
+
+    private static void Apply(float linear)
+    {
+        var mixer = Mixer;
+        if (!mixer)
+        {
+            Debug.LogWarning($"[MasterVolume] AudioMixer '{MixerResourcePath}' not found under a Resources/ folder.");
+            return;
+        }
+        if (!mixer.SetFloat(ExposedParam, LinearToDb(linear)))
+            Debug.LogWarning($"[MasterVolume] Exposed parameter '{ExposedParam}' not found on the AudioMixer.");
+    }
+
+    // Push the saved level before any scene audio starts. SetFloat overrides the exposed parameter
+    // permanently (until ClearFloat), so the start snapshot's authored value is superseded.
+    [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
+    private static void ApplyOnStartup()
+    {
+        Apply(Volume);
+    }
+}