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/GameMode.cs

@@ -0,0 +1,29 @@
+/// <summary>
+/// Base class for a high-level game mode (state) handled by
+/// <see cref="GameModeManager"/>. A mode configures the relevant scene systems
+/// when it becomes active (<see cref="Enter"/>), can clean up when it is left
+/// (<see cref="Exit"/>) and may run per-frame logic in <see cref="Tick"/>.
+///
+/// Modes are plain C# classes (not MonoBehaviours). To add a new mode, derive
+/// from this class and register an instance in GameModeManager.BuildModes().
+/// </summary>
+public abstract class GameMode
+{
+    /// <summary>The owning manager, providing access to the controlled systems.</summary>
+    protected GameModeManager Manager { get; private set; }
+
+    /// <summary>Human-readable name (shown in logs / debug UI).</summary>
+    public abstract string DisplayName { get; }
+
+    /// <summary>Wires the mode to its manager. Called once during registration.</summary>
+    internal void Bind(GameModeManager manager) => Manager = manager;
+
+    /// <summary>Called once when this mode becomes the active mode.</summary>
+    public virtual void Enter() { }
+
+    /// <summary>Called once when this mode stops being the active mode.</summary>
+    public virtual void Exit() { }
+
+    /// <summary>Called every frame while this mode is the active mode.</summary>
+    public virtual void Tick() { }
+}