Implement full mail send/receive: SMSG_SHOW_MAILBOX, CMSG_GET_MAIL_LIST,
SMSG_MAIL_LIST_RESULT, CMSG_SEND_MAIL, SMSG_SEND_MAIL_RESULT, mail take
money/item/delete/mark-as-read, and inbox/compose UI windows.
Fix periodic stuttering in Stormwind caused by synchronous per-line disk
flushes in the logger — remove fileStream.flush() and std::endl, downgrade
high-volume per-packet/per-model/per-texture LOG_INFO to LOG_DEBUG.
Online world entry now shows a progress bar during character model,
terrain, and tile streaming. Fixed loading screen crash from calling
ImGui::NewFrame while a frame was already in progress. Log file is
now truncated on each launch instead of appending.
