game/rendering: drive player stand-state animation from SMSG_STANDSTATE_UPDATE

Add StandStateCallback to GameHandler, fired when the server confirms
a stand state change (SMSG_STANDSTATE_UPDATE). Connect in Application
to map the WoW stand state (0-8) to M2 animation IDs on the player
character model:
  - 0 = Stand → anim 0 (Stand)
  - 1-6 = Sit variants → anim 27 (SitGround)
  - 7 = Dead → anim 1 (Death)
  - 8 = Kneel → anim 72 (Kneel)

Sit and Kneel animations are looped so the held-pose frame stays
visible; Death stays on the final frame.

src/game/game_handler.cpp

@@ -4253,6 +4253,9 @@ void GameHandler::handlePacket(network::Packet& packet) {
                 LOG_INFO("Stand state updated: ", static_cast<int>(standState_),
                          " (", standState_ == 0 ? "stand" : standState_ == 1 ? "sit"
                             : standState_ == 7 ? "dead" : standState_ == 8 ? "kneel" : "other", ")");
+                if (standStateCallback_) {
+                    standStateCallback_(standState_);
+                }
             }
             break;
         case Opcode::SMSG_NEW_TAXI_PATH: