refactor(chat): decompose into modular architecture, add GM commands, fix protocol

- Extract ChatPanel monolith into 15+ focused modules under ui/chat/
  (ChatInput, ChatTabManager, ChatTabCompleter, ChatMarkupParser,
  ChatMarkupRenderer, ChatCommandRegistry, ChatBubbleManager,
  ChatSettings, MacroEvaluator, GameStateAdapter, InputModifierAdapter)
- Split 2700-line chat_panel_commands.cpp into 11 command modules
- Add GM command handling: 190-command data table, dot-prefix interception,
  tab-completion, /gmhelp with category filter
- Fix ChatType enum to match WoW wire protocol (SAY=0x01 not 0x00);
  values 0x00-0x1B shared across Vanilla/TBC/WotLK
- Fix BG_SYSTEM_* values from 82-84 (UB in bitmask shifts) to 0x24-0x26
- Fix infinite Enter key loop after teleport (disable TOGGLE_CHAT repeat,
  add 2-frame input cooldown)
- Add tests: chat_markup_parser, chat_tab_completer, gm_commands,
  macro_evaluator

Signed-off-by: Pavel Okhlopkov <pavel.okhlopkov@flant.com>

include/ui/chat/chat_fwd.hpp

@@ -0,0 +1,37 @@
+// Forward declarations for the chat subsystem.
+// Include this instead of the full headers when only pointers/references are needed.
+// Extracted in Phase 6.6 of chat_panel_ref.md.
+#pragma once
+
+namespace wowee {
+
+namespace game { class GameHandler; }
+
+namespace ui {
+
+class ChatPanel;
+class InventoryScreen;
+class SpellbookScreen;
+class QuestLogScreen;
+
+// Chat subsystem types (under include/ui/chat/)
+class ChatSettings;
+class ChatInput;
+class ChatTabManager;
+class ChatBubbleManager;
+class ChatMarkupParser;
+class ChatMarkupRenderer;
+class ChatCommandRegistry;
+class ChatTabCompleter;
+class CastSequenceTracker;
+class MacroEvaluator;
+class IGameState;
+class IModifierState;
+class IChatCommand;
+
+struct ChatCommandContext;
+struct ChatCommandResult;
+struct UIServices;
+
+} // namespace ui
+} // namespace wowee