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>

src/ui/chat/chat_tab_completer.cpp

@@ -0,0 +1,35 @@
+// ChatTabCompleter — cycling tab-completion state machine.
+// Extracted from ChatPanel tab-completion logic (Phase 5.1).
+#include "ui/chat/chat_tab_completer.hpp"
+
+namespace wowee { namespace ui {
+
+void ChatTabCompleter::startCompletion(const std::string& prefix,
+                                       std::vector<std::string> candidates) {
+    prefix_ = prefix;
+    matches_ = std::move(candidates);
+    matchIdx_ = matches_.empty() ? -1 : 0;
+}
+
+bool ChatTabCompleter::next() {
+    if (matches_.empty()) return false;
+    ++matchIdx_;
+    if (matchIdx_ >= static_cast<int>(matches_.size()))
+        matchIdx_ = 0;
+    return true;
+}
+
+std::string ChatTabCompleter::getCurrentMatch() const {
+    if (matchIdx_ < 0 || matchIdx_ >= static_cast<int>(matches_.size()))
+        return "";
+    return matches_[matchIdx_];
+}
+
+void ChatTabCompleter::reset() {
+    prefix_.clear();
+    matches_.clear();
+    matchIdx_ = -1;
+}
+
+} // namespace ui
+} // namespace wowee