Add game mode manager with player-control and paint modes

Introduce an extensible game mode system toggled with Tab:
- Player Control: WASD movement with always-on mouse camera orbit
- Paint: player frozen, left-click paints, right-click holds to orbit

Add GameMode base class, GameModeManager, and the two concrete modes.
Refactor controlled scripts to expose clean state flags instead of
handling input themselves: ThirdPersonOrbitInput.LookMode,
PlayerCharacterController.ControlEnabled, MeshPainter.PaintingEnabled.

Also fix camera zoom: remove the obsolete /120 wheel normalization so
one scroll notch maps to one zoomSpeed step under the new Input System.

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

Assets/Scripts/PlayerControlMode.cs

@@ -0,0 +1,20 @@
+/// <summary>
+/// Mode 1: the player walks with WASD/Space and the camera orbits freely with
+/// the mouse (cursor locked/hidden). Mesh painting is disabled.
+/// </summary>
+public class PlayerControlMode : GameMode
+{
+    public override string DisplayName => "Player Control";
+
+    public override void Enter()
+    {
+        if (Manager.Player != null)
+            Manager.Player.ControlEnabled = true;
+
+        if (Manager.CameraInput != null)
+            Manager.CameraInput.LookMode = ThirdPersonOrbitInput.OrbitLookMode.AlwaysActive;
+
+        if (Manager.Painter != null)
+            Manager.Painter.PaintingEnabled = false;
+    }
+}