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_tab_completer.hpp

@@ -0,0 +1,53 @@
+// ChatTabCompleter — cycling tab-completion state machine.
+// Extracted from scattered vars in ChatPanel (Phase 5.1).
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace wowee {
+namespace ui {
+
+/**
+ * Stateful tab-completion engine.
+ *
+ * The caller gathers candidates and calls startCompletion() or cycle().
+ * The completer stores the prefix, sorted matches, and current index.
+ */
+class ChatTabCompleter {
+public:
+    /**
+     * Start a new completion session with the given candidates.
+     * Resets the index to 0.
+     */
+    void startCompletion(const std::string& prefix, std::vector<std::string> candidates);
+
+    /**
+     * Cycle to the next match. Returns true if there are matches.
+     * If the prefix changed, the caller should call startCompletion() instead.
+     */
+    bool next();
+
+    /** Get the current match, or "" if no matches. */
+    std::string getCurrentMatch() const;
+
+    /** Get the number of matches in the current session. */
+    size_t matchCount() const { return matches_.size(); }
+
+    /** Check if a completion session is active. */
+    bool isActive() const { return matchIdx_ >= 0; }
+
+    /** Get the current prefix. */
+    const std::string& getPrefix() const { return prefix_; }
+
+    /** Reset the completer (e.g. on text change or arrow key). */
+    void reset();
+
+private:
+    std::string prefix_;
+    std::vector<std::string> matches_;
+    int matchIdx_ = -1;
+};
+
+} // namespace ui
+} // namespace wowee