Add mailbox system and fix logging performance stutter

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.

include/game/opcode_table.hpp

@@ -365,6 +365,19 @@ enum class LogicalOpcode : uint16_t {
     CMSG_CHANNEL_LIST,
     SMSG_CHANNEL_LIST,
 
+    // ---- Mail ----
+    SMSG_SHOW_MAILBOX,
+    CMSG_GET_MAIL_LIST,
+    SMSG_MAIL_LIST_RESULT,
+    CMSG_SEND_MAIL,
+    SMSG_SEND_MAIL_RESULT,
+    CMSG_MAIL_TAKE_MONEY,
+    CMSG_MAIL_TAKE_ITEM,
+    CMSG_MAIL_DELETE,
+    CMSG_MAIL_MARK_AS_READ,
+    SMSG_RECEIVED_MAIL,
+    MSG_QUERY_NEXT_MAIL_TIME,
+
     // Sentinel
     COUNT
 };