feat: add PvP honor credit toast on honorable kill

SMSG_PVP_CREDIT previously only wrote a system chat message.  Now it
also fires a new PvpHonorCallback, which game_screen.cpp uses to push
a compact dark-red toast at the top-right of the screen showing
"⚔ +N Honor" with a 3.5 s lifetime and smooth fade in/out.

src/game/game_handler.cpp

@@ -1882,6 +1882,9 @@ void GameHandler::handlePacket(network::Packet& packet) {
                          std::dec, " rank=", rank);
                 std::string msg = "You gain " + std::to_string(honor) + " honor points.";
                 addSystemChatMessage(msg);
+                if (pvpHonorCallback_) {
+                    pvpHonorCallback_(honor, victimGuid, rank);
+                }
             }
             break;
         }