Upgrade to C++20 and fix all compilation warnings

- Upgrade from C++17 to C++20
- Remove unused helper functions (selectSpawnPreset, parseVec3Csv, parseYawPitchCsv)
- Mark unused parameters with [[maybe_unused]] attribute
- Remove unused variables (nameColor, currentRace, steppingUp, steppingDown, awayFromWallMotion)
- Fix all -Wunused-* warnings

Build now completes with zero warnings.

src/ui/ui_manager.cpp

@@ -76,7 +76,7 @@ void UIManager::shutdown() {
     LOG_INFO("UI manager shutdown");
 }
 
-void UIManager::update(float deltaTime) {
+void UIManager::update([[maybe_unused]] float deltaTime) {
     if (!imguiInitialized) return;
 
     // Start ImGui frame