rendering/game: sync camera sit state from server-confirmed stand state

Add CameraController::setSitting() and call it from the StandStateCallback
so the camera blocks movement when the server confirms the player is
sitting or kneeling (stand states 1-6, 8). This prevents the player
from sliding across the ground after sitting.

Death (state 7) deliberately leaves sitting=false so the player can
still respawn/move after death without input being blocked.

include/rendering/camera_controller.hpp

@@ -82,6 +82,7 @@ public:
     bool isSwimming() const { return swimming; }
     bool isInsideWMO() const { return cachedInsideWMO; }
     void setGrounded(bool g) { grounded = g; }
+    void setSitting(bool s) { sitting = s; }
     bool isOnTaxi() const { return externalFollow_; }
     const glm::vec3* getFollowTarget() const { return followTarget; }
     glm::vec3* getFollowTargetMutable() { return followTarget; }