refactor: extract guidToUnitId/getQuestTitle helpers and misc cleanup

- Extract guidToUnitId(), getQuestTitle(), findQuestLogEntry() helpers
  to replace 14 duplicated GUID-to-unitId patterns and 7 quest log
  search patterns in game_handler.cpp
- Remove duplicate #include in renderer.cpp
- Remove commented-out model cleanup code in terrain_manager.cpp
- Replace C-style casts with static_cast in auth and transport code

src/rendering/renderer.cpp

@@ -33,7 +33,6 @@
 #include "pipeline/asset_manager.hpp"
 #include "pipeline/dbc_loader.hpp"
 #include "pipeline/dbc_layout.hpp"
-#include "pipeline/m2_loader.hpp"
 #include "pipeline/wmo_loader.hpp"
 #include "pipeline/adt_loader.hpp"
 #include "pipeline/terrain_mesh.hpp"

src/rendering/terrain_manager.cpp

@@ -2325,17 +2325,6 @@ void TerrainManager::streamTiles() {
     }
 
     if (!tilesToUnload.empty()) {
-        // Don't clean up models during streaming - keep them in VRAM for performance
-        // Modern GPUs have 8-16GB VRAM, models are only ~hundreds of MB
-        // Cleanup can be done manually when memory pressure is detected
-        // NOTE: Disabled permanent model cleanup to leverage modern VRAM capacity
-        // if (m2Renderer) {
-        //     m2Renderer->cleanupUnusedModels();
-        // }
-        // if (wmoRenderer) {
-        //     wmoRenderer->cleanupUnusedModels();
-        // }
-
         LOG_INFO("Unloaded ", tilesToUnload.size(), " distant tiles, ",
                  loadedTiles.size(), " remain (models kept in VRAM)");
     }