Add persistent global mouse sensitivity setting with slider

MouseSensitivity is a static, PlayerPrefs-backed multiplier (default 1,
clamped 0.1-4.0) that scales the mouse-look delta in the FreeCam,
FirstPerson and ThirdPerson controllers. Writes flush immediately so the
value survives a hard build exit. MouseSensitivitySlider binds the
Settings-panel slider to it (range read from the inspector) and mirrors
the value into its label, syncing on panel open via OnEnable.

Main.unity now contains the session-manager canvas directly (the
Online Session Manager prefab was unpacked) and wires the slider binder.

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

Assets/Scripts/MouseSensitivity.cs

@@ -0,0 +1,46 @@
+using System;
+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.
+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;
+
+    /// Fires whenever the multiplier changes (UI may run before the avatar spawns).
+    public static event Action<float> Changed;
+
+    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();
+            Changed?.Invoke(v);
+        }
+    }
+}