physics: implement knockback simulation from SMSG_MOVE_KNOCK_BACK

Previously the handler ACKed with current position and ignored the
velocity fields entirely (vcos/vsin/hspeed/vspeed were [[maybe_unused]]).
The server expects the client to fly through the air on knockback — without
simulation the player stays in place while the server models them as airborne,
causing position desync and rubberbanding.

Changes:
- CameraController: add applyKnockBack(vcos, vsin, hspeed, vspeed)
  that sets knockbackHorizVel_ and launches verticalVelocity = -vspeed
  (server sends vspeed as negative for upward launches, matching TrinityCore)
- Physics loop: each tick adds knockbackHorizVel_ to targetPos then applies
  exponential drag (KNOCKBACK_HORIZ_DRAG=4.5/s) until velocity < 0.05 u/s
- GameHandler: parse all four fields, add KnockBackCallback, call it for
  the local player so the camera controller receives the impulse
- Application: register the callback — routes server knockback to physics

The existing ACK path is unchanged; the server gets position confirmation
as before while the client now actually simulates the trajectory.

src/core/application.cpp

@@ -636,6 +636,11 @@ void Application::setState(AppState newState) {
                         renderer->triggerMeleeSwing();
                     }
                 });
+                gameHandler->setKnockBackCallback([this](float vcos, float vsin, float hspeed, float vspeed) {
+                    if (renderer && renderer->getCameraController()) {
+                        renderer->getCameraController()->applyKnockBack(vcos, vsin, hspeed, vspeed);
+                    }
+                });
             }
             // Load quest marker models
             loadQuestMarkerModels();