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/FreeCamController.cs

@@ -109,7 +109,7 @@ public class FreeCamController : MonoBehaviour
 
     void LookTick()
     {
-        Vector2 delta = ReadMouseDelta();
+        Vector2 delta = ReadMouseDelta() * MouseSensitivity.Multiplier;
         _yaw += delta.x * horizontalSensitivity;
         float vs = verticalSensitivity * (invertY ? 1f : -1f);
         _pitch = Mathf.Clamp(_pitch + delta.y * vs, -maxPitch, maxPitch);