feat: add SavedVariables persistence for Lua addons

Addons can now persist data across sessions using the standard WoW
SavedVariables pattern:

1. Declare in .toc: ## SavedVariables: MyAddonDB
2. Use the global in Lua: MyAddonDB = MyAddonDB or {default = true}
3. Data is automatically saved on logout and restored on next login

Implementation:
- TocFile::getSavedVariables() parses comma-separated variable names
- LuaEngine::loadSavedVariables() executes saved .lua file to restore globals
- LuaEngine::saveSavedVariables() serializes Lua tables/values to valid Lua
- Serializer handles tables (nested), strings, numbers, booleans, nil
- Save triggered on PLAYER_LEAVING_WORLD and AddonManager::shutdown()
- Files stored as <AddonDir>/<AddonName>.lua.saved

Updated HelloWorld addon to track login count across sessions.

include/addons/addon_manager.hpp

@@ -25,11 +25,14 @@ public:
     LuaEngine* getLuaEngine() { return &luaEngine_; }
     bool isInitialized() const { return luaEngine_.isInitialized(); }
 
+    void saveAllSavedVariables();
+
 private:
     LuaEngine luaEngine_;
     std::vector<TocFile> addons_;
 
     bool loadAddon(const TocFile& addon);
+    std::string getSavedVariablesPath(const TocFile& addon) const;
 };
 
 } // namespace wowee::addons