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/PaintMode.cs

@@ -0,0 +1,21 @@
+/// <summary>
+/// Mode 2: the player stands still and the mesh is painted with the left mouse
+/// button. The camera orbits only while the right mouse button is held, so
+/// dragging with the left button paints instead of rotating the view.
+/// </summary>
+public class PaintMode : GameMode
+{
+    public override string DisplayName => "Paint";
+
+    public override void Enter()
+    {
+        if (Manager.Player != null)
+            Manager.Player.ControlEnabled = false;
+
+        if (Manager.CameraInput != null)
+            Manager.CameraInput.LookMode = ThirdPersonOrbitInput.OrbitLookMode.HoldRightMouseButton;
+
+        if (Manager.Painter != null)
+            Manager.Painter.PaintingEnabled = true;
+    }
+}