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/PlayerCharacterController.cs

@@ -71,6 +71,13 @@ public class PlayerCharacterController : MonoBehaviour
     [Range(0f, 1f)]
     public float slideControl = 0.3f;
 
+    /// <summary>
+    /// When false, movement/jump input is ignored (the player stands still), but
+    /// gravity and ground contact keep running so the character stays grounded.
+    /// Toggled by the GameModeManager (e.g. disabled in paint mode).
+    /// </summary>
+    public bool ControlEnabled { get; set; } = true;
+
     private CharacterController _controller;
     private float _verticalVelocity;
 
@@ -92,21 +99,26 @@ public class PlayerCharacterController : MonoBehaviour
         float h = 0f, v = 0f;
         bool jumpPressed = false;
 
-#if ENABLE_INPUT_SYSTEM
-        var keyboard = Keyboard.current;
-        if (keyboard != null)
+        // Skip input entirely when control is disabled (e.g. paint mode); gravity
+        // and grounding below still run so the player stays planted on the ground.
+        if (ControlEnabled)
         {
-            if (keyboard.aKey.isPressed) h -= 1f;
-            if (keyboard.dKey.isPressed) h += 1f;
-            if (keyboard.sKey.isPressed) v -= 1f;
-            if (keyboard.wKey.isPressed) v += 1f;
-            jumpPressed = keyboard.spaceKey.wasPressedThisFrame;
-        }
+#if ENABLE_INPUT_SYSTEM
+            var keyboard = Keyboard.current;
+            if (keyboard != null)
+            {
+                if (keyboard.aKey.isPressed) h -= 1f;
+                if (keyboard.dKey.isPressed) h += 1f;
+                if (keyboard.sKey.isPressed) v -= 1f;
+                if (keyboard.wKey.isPressed) v += 1f;
+                jumpPressed = keyboard.spaceKey.wasPressedThisFrame;
+            }
 #else
-        h = Input.GetAxisRaw("Horizontal");
-        v = Input.GetAxisRaw("Vertical");
-        jumpPressed = Input.GetButtonDown("Jump");
+            h = Input.GetAxisRaw("Horizontal");
+            v = Input.GetAxisRaw("Vertical");
+            jumpPressed = Input.GetButtonDown("Jump");
 #endif
+        }
 
         // --- Build the horizontal move direction ----------------------------
         Vector3 inputDir = new Vector3(h, 0f, v);