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/i_chat_command.hpp

@@ -0,0 +1,57 @@
+// IChatCommand — interface for all slash commands.
+// Phase 3.1 of chat_panel_ref.md.
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace wowee {
+
+// Forward declarations
+namespace game { class GameHandler; }
+namespace ui { struct UIServices; class ChatPanel; }
+
+namespace ui {
+
+/**
+ * Context passed to every command's execute() method.
+ * Provides everything a command needs without coupling to ChatPanel.
+ */
+struct ChatCommandContext {
+    game::GameHandler&  gameHandler;
+    UIServices&         services;
+    ChatPanel&          panel;          // for input buffer access, macro state
+    std::string         args;           // everything after "/cmd "
+    std::string         fullCommand;    // the original command name (lowercase)
+};
+
+/**
+ * Result returned by a command to tell the dispatcher what to do next.
+ */
+struct ChatCommandResult {
+    bool handled = true;           // false → command not recognized, fall through
+    bool clearInput = true;        // clear the input buffer after execution
+};
+
+/**
+ * Interface for all chat slash commands.
+ *
+ * Adding a new command = create a class implementing this interface,
+ * register it in ChatCommandRegistry. Zero edits to existing code. (OCP)
+ */
+class IChatCommand {
+public:
+    virtual ~IChatCommand() = default;
+
+    /** Execute the command. */
+    virtual ChatCommandResult execute(ChatCommandContext& ctx) = 0;
+
+    /** Return all aliases for this command (e.g. {"w", "whisper", "tell", "t"}). */
+    virtual std::vector<std::string> aliases() const = 0;
+
+    /** Optional help text shown by /help. */
+    virtual std::string helpText() const { return ""; }
+};
+
+} // namespace ui
+} // namespace wowee