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.

include/game/game_handler.hpp

@@ -693,6 +693,11 @@ public:
     using WorldEntryCallback = std::function<void(uint32_t mapId, float x, float y, float z, bool isInitialEntry)>;
     void setWorldEntryCallback(WorldEntryCallback cb) { worldEntryCallback_ = std::move(cb); }
 
+    // Knockback callback: called when server sends SMSG_MOVE_KNOCK_BACK for the player.
+    // Parameters: vcos, vsin (render-space direction), hspeed, vspeed (raw from packet).
+    using KnockBackCallback = std::function<void(float vcos, float vsin, float hspeed, float vspeed)>;
+    void setKnockBackCallback(KnockBackCallback cb) { knockBackCallback_ = std::move(cb); }
+
     // Unstuck callback (resets player Z to floor height)
     using UnstuckCallback = std::function<void()>;
     void setUnstuckCallback(UnstuckCallback cb) { unstuckCallback_ = std::move(cb); }
@@ -1812,6 +1817,7 @@ private:
 
     // ---- Phase 3: Spells ----
     WorldEntryCallback worldEntryCallback_;
+    KnockBackCallback knockBackCallback_;
     UnstuckCallback unstuckCallback_;
     UnstuckCallback unstuckGyCallback_;
     UnstuckCallback unstuckHearthCallback_;